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How to add a simple message by 'echo' in invoice pdf format in Magento2?


I have generated an invoice slip from magento2 sales order's invoice option and got the screenshot for the reference, see the screenshot: https://prnt.sc/tje4zn

This is the file path for invoice pdf: /vendor/magento/module-sales/Model/Order/Pdf/AbstractPdf.php and below is the file code,

<?php
namespace Magento\Sales\Model\Order\Pdf;
use Magento\Framework\App\Filesystem\DirectoryList;
/**
 * Sales Order PDF abstract model
 */
abstract class AbstractPdf extends \Magento\Framework\DataObject
{
    public $y;
    protected $_renderers = [];
    const XML_PATH_SALES_PDF_INVOICE_PUT_ORDER_ID = 'sales_pdf/invoice/put_order_id';

    const XML_PATH_SALES_PDF_SHIPMENT_PUT_ORDER_ID = 'sales_pdf/shipment/put_order_id';

    const XML_PATH_SALES_PDF_CREDITMEMO_PUT_ORDER_ID = 'sales_pdf/creditmemo/put_order_id';

    protected $_pdf;
    abstract public function getPdf();
    protected $_paymentData;
    protected $string;
    protected $_localeDate;
    protected $_scopeConfig;
    protected $_mediaDirectory;
    protected $_rootDirectory;
    protected $_pdfConfig;
    protected $_pdfTotalFactory;
    protected $_pdfItemsFactory;
    protected $inlineTranslation;
    protected $addressRenderer;
    public function __construct(
        \Magento\Payment\Helper\Data $paymentData,
        \Magento\Framework\Stdlib\StringUtils $string,
        \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig,
        \Magento\Framework\Filesystem $filesystem,
        Config $pdfConfig,
        \Magento\Sales\Model\Order\Pdf\Total\Factory $pdfTotalFactory,
        \Magento\Sales\Model\Order\Pdf\ItemsFactory $pdfItemsFactory,
        \Magento\Framework\Stdlib\DateTime\TimezoneInterface $localeDate,
        \Magento\Framework\Translate\Inline\StateInterface $inlineTranslation,
        \Magento\Sales\Model\Order\Address\Renderer $addressRenderer,
        array $data = []
    ) {
        $this->addressRenderer = $addressRenderer;
        $this->_paymentData = $paymentData;
        $this->_localeDate = $localeDate;
        $this->string = $string;
        $this->_scopeConfig = $scopeConfig;
        $this->_mediaDirectory = $filesystem->getDirectoryWrite(DirectoryList::MEDIA);
        $this->_rootDirectory = $filesystem->getDirectoryRead(DirectoryList::ROOT);
        $this->_pdfConfig = $pdfConfig;
        $this->_pdfTotalFactory = $pdfTotalFactory;
        $this->_pdfItemsFactory = $pdfItemsFactory;
        $this->inlineTranslation = $inlineTranslation;
        parent::__construct($data);
    }
    public function widthForStringUsingFontSize($string, $font, $fontSize)
    {
        $drawingString = '"libiconv"' == ICONV_IMPL ? iconv(
            'UTF-8',
            'UTF-16BE//IGNORE',
            $string
        ) : @iconv(
            'UTF-8',
            'UTF-16BE',
            $string
        );

        $characters = [];
        for ($i = 0; $i < strlen($drawingString); $i++) {
            $characters[] = ord($drawingString[$i++]) << 8 | ord($drawingString[$i]);
        }
        $glyphs = $font->glyphNumbersForCharacters($characters);
        $widths = $font->widthsForGlyphs($glyphs);
        $stringWidth = array_sum($widths) / $font->getUnitsPerEm() * $fontSize;
        return $stringWidth;
    }
    public function getAlignRight($string, $x, $columnWidth, \Zend_Pdf_Resource_Font $font, $fontSize, $padding = 5)
    {
        $width = $this->widthForStringUsingFontSize($string, $font, $fontSize);
        return $x + $columnWidth - $width - $padding;
    }
    public function getAlignCenter($string, $x, $columnWidth, \Zend_Pdf_Resource_Font $font, $fontSize)
    {
        $width = $this->widthForStringUsingFontSize($string, $font, $fontSize);
        return $x + round(($columnWidth - $width) / 2);
    }
    protected function insertLogo(&$page, $store = null)
    {
        $this->y = $this->y ? $this->y : 815;
        $image = $this->_scopeConfig->getValue(
            'sales/identity/logo',
            \Magento\Store\Model\ScopeInterface::SCOPE_STORE,
            $store
        );
        if ($image) {
            $imagePath = '/sales/store/logo/' . $image;
            if ($this->_mediaDirectory->isFile($imagePath)) {
                $image = \Zend_Pdf_Image::imageWithPath($this->_mediaDirectory->getAbsolutePath($imagePath));
                $top = 830;
                //top border of the page
                $widthLimit = 270;
                //half of the page width
                $heightLimit = 270;
                //assuming the image is not a "skyscraper"
                $width = $image->getPixelWidth();
                $height = $image->getPixelHeight();

                //preserving aspect ratio (proportions)
                $ratio = $width / $height;
                if ($ratio > 1 && $width > $widthLimit) {
                    $width = $widthLimit;
                    $height = $width / $ratio;
                } elseif ($ratio < 1 && $height > $heightLimit) {
                    $height = $heightLimit;
                    $width = $height * $ratio;
                } elseif ($ratio == 1 && $height > $heightLimit) {
                    $height = $heightLimit;
                    $width = $widthLimit;
                }

                $y1 = $top - $height;
                $y2 = $top;
                $x1 = 25;
                $x2 = $x1 + $width;

                //coordinates after transformation are rounded by Zend
                $page->drawImage($image, $x1, $y1, $x2, $y2);

                $this->y = $y1 - 10;
            }
        }
    }
    protected function _formatAddress($address)
    {
        $return = [];
        foreach (explode('|', $address) as $str) {
            foreach ($this->string->split($str, 45, true, true) as $part) {
                if (empty($part)) {
                    continue;
                }
                $return[] = $part;
            }
        }
        return $return;
    }
    protected function _calcAddressHeight($address)
    {
        $y = 0;
        foreach ($address as $value) {
            if ($value !== '') {
                $text = [];
                foreach ($this->string->split($value, 55, true, true) as $_value) {
                    $text[] = $_value;
                }
                foreach ($text as $part) {
                    $y += 15;
                }
            }
        }
        return $y;
    }
    protected function insertOrder(&$page, $obj, $putOrderId = true)
    {
        if ($obj instanceof \Magento\Sales\Model\Order) {
            $shipment = null;
            $order = $obj;
        } elseif ($obj instanceof \Magento\Sales\Model\Order\Shipment) {
            $shipment = $obj;
            $order = $shipment->getOrder();
        }

        $this->y = $this->y ? $this->y : 815;
        $top = $this->y;

        $page->setFillColor(new \Zend_Pdf_Color_GrayScale(0.45));
        $page->setLineColor(new \Zend_Pdf_Color_GrayScale(0.45));
        $page->drawRectangle(25, $top, 570, $top - 55);
        $page->setFillColor(new \Zend_Pdf_Color_GrayScale(1));
        $this->setDocHeaderCoordinates([25, $top, 570, $top - 55]);
        $this->_setFontRegular($page, 10);

        if ($putOrderId) {
            $page->drawText(__('Order No # ') . $order->getRealOrderId(), 35, $top -= 30, 'UTF-8');
            $top +=15;
        }

        $top -=30;
        $page->drawText(
            __('Order Date: ') .
            $this->_localeDate->formatDate(
                $this->_localeDate->scopeDate(
                    $order->getStore(),
                    $order->getCreatedAt(),
                    true
                ),
                \IntlDateFormatter::MEDIUM,
                false
            ),
            35,
            $top,
            'UTF-8'
        );

        $top -= 10;
        $page->setFillColor(new \Zend_Pdf_Color_Rgb(0.93, 0.92, 0.92));
        $page->setLineColor(new \Zend_Pdf_Color_GrayScale(0.5));
        $page->setLineWidth(0.5);
        $page->drawRectangle(25, $top, 275, $top - 25);
        $page->drawRectangle(275, $top, 570, $top - 25);    
        /* Calculate blocks info */    
        /* Billing Address */
        $billingAddress = $this->_formatAddress($this->addressRenderer->format($order->getBillingAddress(), 'pdf'));    
        /* Payment */
        $paymentInfo = $this->_paymentData->getInfoBlock($order->getPayment())->setIsSecureMode(true)->toPdf();
        $paymentInfo = htmlspecialchars_decode($paymentInfo, ENT_QUOTES);
        $payment = explode('{{pdf_row_separator}}', $paymentInfo);
        foreach ($payment as $key => $value) {
            if (strip_tags(trim($value)) == '') {
                unset($payment[$key]);
            }
        }
        reset($payment);    
        /* Shipping Address and Method */
        if (!$order->getIsVirtual()) {
            /* Shipping Address */
            $shippingAddress = $this->_formatAddress($this->addressRenderer->format($order->getShippingAddress(), 'pdf'));
            $shippingMethod = $order->getShippingDescription();
        }
        $page->setFillColor(new \Zend_Pdf_Color_GrayScale(0));
        $this->_setFontBold($page, 12);
        $page->drawText(__('Billed to:'), 35, $top - 15, 'UTF-8');

        if (!$order->getIsVirtual()) {
            $page->drawText(__('Shipped/Delivery to:'), 285, $top - 15, 'UTF-8');
        } else {
            $page->drawText(__('Payment Method:'), 285, $top - 15, 'UTF-8');
        }

        $addressesHeight = $this->_calcAddressHeight($billingAddress);
        if (isset($shippingAddress)) {
            $addressesHeight = max($addressesHeight, $this->_calcAddressHeight($shippingAddress));
        }    
        $page->setFillColor(new \Zend_Pdf_Color_GrayScale(1));
        $page->drawRectangle(25, $top - 25, 570, $top - 33 - $addressesHeight);
        $page->setFillColor(new \Zend_Pdf_Color_GrayScale(0));
        $this->_setFontRegular($page, 10);
        $this->y = $top - 40;
        $addressesStartY = $this->y;    
        foreach ($billingAddress as $value) {
            if ($value !== '') {
                $text = [];
                foreach ($this->string->split($value, 45, true, true) as $_value) {
                    $text[] = $_value;
                }
                foreach ($text as $part) {
                    $page->drawText(strip_tags(ltrim($part)), 35, $this->y, 'UTF-8');
                    $this->y -= 15;
                }
            }
        }    
        $addressesEndY = $this->y;    
        if (!$order->getIsVirtual()) {
            $this->y = $addressesStartY;
            foreach ($shippingAddress as $value) {
                if ($value !== '') {
                    $text = [];
                    foreach ($this->string->split($value, 45, true, true) as $_value) {
                        $text[] = $_value;
                    }
                    foreach ($text as $part) {
                        $page->drawText(strip_tags(ltrim($part)), 285, $this->y, 'UTF-8');
                        $this->y -= 15;
                    }
                }
            }    
            $addressesEndY = min($addressesEndY, $this->y);
            $this->y = $addressesEndY;    
            $page->setFillColor(new \Zend_Pdf_Color_Rgb(0.93, 0.92, 0.92));
            $page->setLineWidth(0.5);
            $page->drawRectangle(25, $this->y, 275, $this->y - 25);
            $page->drawRectangle(275, $this->y, 570, $this->y - 25);    
            $this->y -= 15;
            $this->_setFontBold($page, 12);
            $page->setFillColor(new \Zend_Pdf_Color_GrayScale(0));
            $page->drawText(__('Payment Method'), 35, $this->y, 'UTF-8');
            $page->drawText(__('Shipping Method:'), 285, $this->y, 'UTF-8');    
            $this->y -= 10;
            $page->setFillColor(new \Zend_Pdf_Color_GrayScale(1));    
            $this->_setFontRegular($page, 10);
            $page->setFillColor(new \Zend_Pdf_Color_GrayScale(0));    
            $paymentLeft = 35;
            $yPayments = $this->y - 15;
        } else {
            $yPayments = $addressesStartY;
            $paymentLeft = 285;
        }    
        foreach ($payment as $value) {
            if (trim($value) != '') {
                //Printing "Payment Method" lines
                $value = preg_replace('/<br[^>]*>/i', "\n", $value);
                foreach ($this->string->split($value, 45, true, true) as $_value) {
                    $page->drawText(strip_tags(trim($_value)), $paymentLeft, $yPayments, 'UTF-8');
                    $yPayments -= 15;
                }
            }
        }    
        if ($order->getIsVirtual()) {
            // replacement of Shipments-Payments rectangle block
            $yPayments = min($addressesEndY, $yPayments);
            $page->drawLine(25, $top - 25, 25, $yPayments);
            $page->drawLine(570, $top - 25, 570, $yPayments);
            $page->drawLine(25, $yPayments, 570, $yPayments);    
            $this->y = $yPayments - 15;
        } else {
            $topMargin = 15;
            $methodStartY = $this->y;
            $this->y -= 15;    
            foreach ($this->string->split($shippingMethod, 45, true, true) as $_value) {
                $page->drawText(strip_tags(trim($_value)), 285, $this->y, 'UTF-8');
                $this->y -= 15;
            }    
            $yShipments = $this->y;
            $totalShippingChargesText = "(" . __(
                    'Total Shipping Charges'
                ) . " " . $order->formatPriceTxt(
                    $order->getShippingAmount()
                ) . ")";    
            $page->drawText($totalShippingChargesText, 285, $yShipments - $topMargin, 'UTF-8');
            $yShipments -= $topMargin + 10;    
            $tracks = [];
            if ($shipment) {
                $tracks = $shipment->getAllTracks();
            }
            if (count($tracks)) {
                $page->setFillColor(new \Zend_Pdf_Color_Rgb(0.93, 0.92, 0.92));
                $page->setLineWidth(0.5);
                $page->drawRectangle(285, $yShipments, 510, $yShipments - 10);
                $page->drawLine(400, $yShipments, 400, $yShipments - 10);
                //$page->drawLine(510, $yShipments, 510, $yShipments - 10);    
                $this->_setFontRegular($page, 9);
                $page->setFillColor(new \Zend_Pdf_Color_GrayScale(0));
                //$page->drawText(__('Carrier'), 290, $yShipments - 7 , 'UTF-8');
                $page->drawText(__('Title'), 290, $yShipments - 7, 'UTF-8');
                $page->drawText(__('Number'), 410, $yShipments - 7, 'UTF-8');    
                $yShipments -= 20;
                $this->_setFontRegular($page, 8);
                foreach ($tracks as $track) {
                    $maxTitleLen = 45;
                    $endOfTitle = strlen($track->getTitle()) > $maxTitleLen ? '...' : '';
                    $truncatedTitle = substr($track->getTitle(), 0, $maxTitleLen) . $endOfTitle;
                    $page->drawText($truncatedTitle, 292, $yShipments, 'UTF-8');
                    $page->drawText($track->getNumber(), 410, $yShipments, 'UTF-8');
                    $yShipments -= $topMargin - 5;
                }
            } else {
                $yShipments -= $topMargin - 5;
            }    
            $currentY = min($yPayments, $yShipments);
            $page->drawLine(25, $methodStartY, 25, $currentY);
            $page->drawLine(25, $currentY, 570, $currentY);
            $page->drawLine(570, $currentY, 570, $methodStartY);    
            $this->y = $currentY;
            $this->y -= 15;
        }
    }
    public function insertDocumentNumber(\Zend_Pdf_Page $page, $text)
    {
        $page->setFillColor(new \Zend_Pdf_Color_GrayScale(1));
        $this->_setFontRegular($page, 10);
        $docHeader = $this->getDocHeaderCoordinates();
        $page->drawText($text, 35, $docHeader[1] - 15, 'UTF-8');
    }
    protected function _sortTotalsList($a, $b)
    {
        if (!isset($a['sort_order']) || !isset($b['sort_order'])) {
            return 0;
        }

        if ($a['sort_order'] == $b['sort_order']) {
            return 0;
        }

        return $a['sort_order'] > $b['sort_order'] ? 1 : -1;
    }
    protected function _getTotalsList()
    {
        $totals = $this->_pdfConfig->getTotals();
        usort($totals, [$this, '_sortTotalsList']);
        $totalModels = [];
        foreach ($totals as $totalInfo) {
            $class = empty($totalInfo['model']) ? null : $totalInfo['model'];
            $totalModel = $this->_pdfTotalFactory->create($class);
            $totalModel->setData($totalInfo);
            $totalModels[] = $totalModel;
        }    
        return $totalModels;
    }
    protected function insertTotals($page, $source)
    {
        $order = $source->getOrder();
        $totals = $this->_getTotalsList();
        $lineBlock = ['lines' => [], 'height' => 15];
        foreach ($totals as $total) {
            $total->setOrder($order)->setSource($source);

            if ($total->canDisplay()) {
                $total->setFontSize(10);
                foreach ($total->getTotalsForDisplay() as $totalData) {
                    $lineBlock['lines'][] = [
                        [
                            'text' => $totalData['label'],
                            'feed' => 475,
                            'align' => 'right',
                            'font_size' => $totalData['font_size'],
                            'font' => 'bold',
                        ],
                        [
                            'text' => $totalData['amount'],
                            'feed' => 565,
                            'align' => 'right',
                            'font_size' => $totalData['font_size'],
                            'font' => 'bold'
                        ],
                    ];
                }
            }
        }    
        $this->y -= 20;
        $page = $this->drawLineBlocks($page, [$lineBlock]);
        return $page;
    }
    protected function _parseItemDescription($item)
    {
        $matches = [];
        $description = $item->getDescription();
        if (preg_match_all('/<li.*?>(.*?)<\/li>/i', $description, $matches)) {
            return $matches[1];
        }    
        return [$description];
    }    
    protected function _beforeGetPdf()
    {
        $this->inlineTranslation->suspend();
    }

    protected function _afterGetPdf()
    {
        $this->inlineTranslation->resume();
    }
    protected function _formatOptionValue($value, $order)
    {
        $resultValue = '';
        if (is_array($value)) {
            if (isset($value['qty'])) {
                $resultValue .= sprintf('%d', $value['qty']) . ' x ';
            }    
            $resultValue .= $value['title'];

            if (isset($value['price'])) {
                $resultValue .= " " . $order->formatPrice($value['price']);
            }
            return $resultValue;
        } else {
            return $value;
        }
    }
    protected function _initRenderer($type)
    {
        $rendererData = $this->_pdfConfig->getRenderersPerProduct($type);
        foreach ($rendererData as $productType => $renderer) {
            $this->_renderers[$productType] = ['model' => $renderer, 'renderer' => null];
        }
    }
    protected function _getRenderer($type)
    {
        if (!isset($this->_renderers[$type])) {
            $type = 'default';
        }    
        if (!isset($this->_renderers[$type])) {
            throw new \Magento\Framework\Exception\LocalizedException(__('We found an invalid renderer model.'));
        }    
        if ($this->_renderers[$type]['renderer'] === null) {
            $this->_renderers[$type]['renderer'] = $this->_pdfItemsFactory->get($this->_renderers[$type]['model']);
        }    
        return $this->_renderers[$type]['renderer'];
    }
    public function getRenderer($type)
    {
        return $this->_getRenderer($type);
    }
    protected function _drawItem(\Magento\Framework\DataObject $item, \Zend_Pdf_Page $page, \Magento\Sales\Model\Order $order)
    {
        $type = $item->getOrderItem()->getProductType();
        $renderer = $this->_getRenderer($type);
        $renderer->setOrder($order);
        $renderer->setItem($item);
        $renderer->setPdf($this);
        $renderer->setPage($page);
        $renderer->setRenderedModel($this);    
        $renderer->draw();
        return $renderer->getPage();
    } 
    protected function _setFontRegular($object, $size = 7)
    {
        $font = \Zend_Pdf_Font::fontWithPath(
            $this->_rootDirectory->getAbsolutePath('lib/internal/GnuFreeFont/FreeSerif.ttf')
        );
        $object->setFont($font, $size);
        return $font;
    }
    protected function _setFontBold($object, $size = 7)
    {
        $font = \Zend_Pdf_Font::fontWithPath(
            $this->_rootDirectory->getAbsolutePath('lib/internal/GnuFreeFont/FreeSerifBold.ttf')
        );
        $object->setFont($font, $size);
        return $font;
    }
    protected function _setFontItalic($object, $size = 7)
    {
        $font = \Zend_Pdf_Font::fontWithPath(
            $this->_rootDirectory->getAbsolutePath('lib/internal/GnuFreeFont/FreeSerifItalic.ttf')
        );
        $object->setFont($font, $size);
        return $font;
    }
    protected function _setPdf(\Zend_Pdf $pdf)
    {
        $this->_pdf = $pdf;
        return $this;
    }
    protected function _getPdf()
    {
        if (!$this->_pdf instanceof \Zend_Pdf) {
            throw new \Magento\Framework\Exception\LocalizedException(__('Please define the PDF object before using.'));
        }

        return $this->_pdf;
    }
    public function newPage(array $settings = [])
    {
        $pageSize = !empty($settings['page_size']) ? $settings['page_size'] : \Zend_Pdf_Page::SIZE_A4;
        $page = $this->_getPdf()->newPage($pageSize);
        $this->_getPdf()->pages[] = $page;
        $this->y = 800;

        return $page;
    }
    public function drawLineBlocks(\Zend_Pdf_Page $page, array $draw, array $pageSettings = [])
    {
        foreach ($draw as $itemsProp) {
            if (!isset($itemsProp['lines']) || !is_array($itemsProp['lines'])) {
                throw new \Magento\Framework\Exception\LocalizedException(
                    __('We don\'t recognize the draw line data. Please define the "lines" array.')
                );
            }
            $lines = $itemsProp['lines'];
            $height = isset($itemsProp['height']) ? $itemsProp['height'] : 10;

            if (empty($itemsProp['shift'])) {
                $shift = 0;
                foreach ($lines as $line) {
                    $maxHeight = 0;
                    foreach ($line as $column) {
                        $lineSpacing = !empty($column['height']) ? $column['height'] : $height;
                        if (!is_array($column['text'])) {
                            $column['text'] = [$column['text']];
                        }
                        $top = 0;
                        foreach ($column['text'] as $part) {
                            $top += $lineSpacing;
                        }

                        $maxHeight = $top > $maxHeight ? $top : $maxHeight;
                    }
                    $shift += $maxHeight;
                }
                $itemsProp['shift'] = $shift;
            }

            if ($this->y - $itemsProp['shift'] < 15) {
                $page = $this->newPage($pageSettings);
            }

            foreach ($lines as $line) {
                $maxHeight = 0;
                foreach ($line as $column) {
                    $font = $this->setFont($page, $column);
                    $fontSize = $column['font_size'];

                    if (!is_array($column['text'])) {
                        $column['text'] = [$column['text']];
                    }

                    $lineSpacing = !empty($column['height']) ? $column['height'] : $height;
                    $top = 0;
                    foreach ($column['text'] as $part) {
                        if ($this->y - $lineSpacing < 15) {
                            $page = $this->newPage($pageSettings);
                            $font = $this->setFont($page, $column);
                            $fontSize = $column['font_size'];
                        }

                        $feed = $column['feed'];
                        $textAlign = empty($column['align']) ? 'left' : $column['align'];
                        $width = empty($column['width']) ? 0 : $column['width'];
                        switch ($textAlign) {
                            case 'right':
                                if ($width) {
                                    $feed = $this->getAlignRight($part, $feed, $width, $font, $fontSize);
                                } else {
                                    $feed = $feed - $this->widthForStringUsingFontSize($part, $font, $fontSize);
                                }
                                break;
                            case 'center':
                                if ($width) {
                                    $feed = $this->getAlignCenter($part, $feed, $width, $font, $fontSize);
                                }
                                break;
                            default:
                                break;
                        }
                        $page->drawText($part, $feed, $this->y - $top, 'UTF-8');
                        $top += $lineSpacing;
                    }

                    $maxHeight = $top > $maxHeight ? $top : $maxHeight;
                }
                $this->y -= $maxHeight;
            }
        }

        return $page;
    }
}

I want to echo simple text message in the end of the file like <?php echo "NOTE: This is not a GST invoice. This is a packing slip only."; ?>

Please help how I can add this message to pdf invoice format as I have also mentioned in the screenshot above.

Many Thanks in Advance.


Solution

  • Assuming you have a custom MyCompany_Invoice module:
    Put following content into the MyCompany/Invoice/etc/di.xml:

    <?xml version="1.0"?>
    <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
        <preference for="Magento\Sales\Model\Order\Pdf\Invoice" type="MyCompany\Invoice\Model\Order\Pdf\InvoicePdf"/>
    </config>
    

    Now create MyCompany\Invoice\Model\Order\Pdf\InvoicePdf.php:

    <?php
    
    namespace MyCompany\Invoice\Model\Order\Pdf;
    
    use \Magento\Sales\Model\Order\Pdf\Invoice;
    
    class InvoicePdf extends Invoice
    {
        /**
         * We only need to override the getPdf of Invoice,
         *  most of this method is copied directly from parent class
         *
         * @param array $invoices
         * @return \Zend_Pdf
         */
        public function getPdf($invoices = []) {
            $this->_beforeGetPdf();
            $this->_initRenderer('invoice');
    
            $pdf = new \Zend_Pdf();
            $this->_setPdf($pdf);
            $style = new \Zend_Pdf_Style();
            $this->_setFontBold($style, 10);
    
            foreach ($invoices as $invoice) {
                if ($invoice->getStoreId()) {
                    $this->_localeResolver->emulate($invoice->getStoreId());
                    $this->_storeManager->setCurrentStore($invoice->getStoreId());
                }
                $page = $this->newPage();
                $order = $invoice->getOrder();
                /* Add image */
                $this->insertLogo($page, $invoice->getStore());
                /* Add address */
                $this->insertAddress($page, $invoice->getStore());
                /* Add head */
                $this->insertOrder(
                    $page,
                    $order,
                    $this->_scopeConfig->isSetFlag(
                        self::XML_PATH_SALES_PDF_INVOICE_PUT_ORDER_ID,
                        \Magento\Store\Model\ScopeInterface::SCOPE_STORE,
                        $order->getStoreId()
                    )
                );
                /* Add document text and number */
                $this->insertDocumentNumber($page, __('Invoice # ') . $invoice->getIncrementId());
                /* Add table */
                $this->_drawHeader($page);
                /* Add body */
                foreach ($invoice->getAllItems() as $item) {
                    if ($item->getOrderItem()->getParentItem()) {
                        continue;
                    }
                    /* Draw item */
                    $this->_drawItem($item, $page, $order);
                    $page = end($pdf->pages);
                }
                /* Add totals */
                $this->insertTotals($page, $invoice);
                if ($invoice->getStoreId()) {
                    $this->_localeResolver->revert();
                }
                // draw custom notice
                $this->drawNotice($page);
            }
            $this->_afterGetPdf();
            return $pdf;
        }
    
        /**
         * draw notice below content
         *
         * @param \Zend_Pdf_Page $page
         */
        protected function drawNotice(\Zend_Pdf_Page $page) {
            $iFontSize = 10;     // font size
            $iColumnWidth = 520; // whole page width
            $iWidthBorder = 260; // half page width
            $sNotice = "NOTE: This is not a GST invoice. This is a packing slip only."; // your message
            $iXCoordinateText = 30;
            $sEncoding = 'UTF-8';
            $this->y -= 10; // move down on page
            try {
                $oFont = $this->_setFontRegular($page, $iFontSize);
                $iXCoordinateText = $this->getAlignCenter($sNotice, $iXCoordinateText, $iColumnWidth, $oFont, $iFontSize);  // center text coordinate
                $page->setLineColor(new \Zend_Pdf_Color_Rgb(1, 0, 0));                                             // red lines
                $iXCoordinateBorder = $iXCoordinateText - 10;                                                               // border is wider than text
                // draw top border
                $page->drawLine($iXCoordinateBorder, $this->y, $iXCoordinateBorder + $iWidthBorder, $this->y);
                // draw text
                $this->y -= 15;                                                                                             // further down
                $page->drawText($sNotice, $iXCoordinateText, $this->y, $sEncoding);
                $this->y -= 10; // further down
                // draw bottom border
                $page->drawLine($iXCoordinateBorder, $this->y, $iXCoordinateBorder + $iWidthBorder, $this->y);
                // draw left border
                $page->drawLine($iXCoordinateBorder, $this->y, $iXCoordinateBorder, $this->y + 25 /* back to first line */);
                // draw right border
                $page->drawLine($iXCoordinateBorder + $iWidthBorder, $this->y, $iXCoordinateBorder + $iWidthBorder, $this->y + 25 /* back to first line */);
                $this->y -= 10;
            } catch (\Exception $exception) {
                // handle
            }
        }
    
        /**
         * Draw header for item table
         *
         * @param \Zend_Pdf_Page $page
         * @return void
         */
        protected function _drawHeader(\Zend_Pdf_Page $page)
        {
            /* Add table head */
            $this->_setFontRegular($page, 10);
            $page->setFillColor(new \Zend_Pdf_Color_Rgb(0.93, 0.92, 0.92));
            $page->setLineColor(new \Zend_Pdf_Color_GrayScale(0.5));
            $page->setLineWidth(0.5);
            $page->drawRectangle(25, $this->y, 570, $this->y - 15);
            $this->y -= 10;
            $page->setFillColor(new \Zend_Pdf_Color_Rgb(0, 0, 0));
    
            //columns headers
            $lines[0][] = ['text' => __('Products'), 'feed' => 35];
    
            $lines[0][] = ['text' => __('SKU'), 'feed' => 290, 'align' => 'right'];
    
            // custom column
            $lines[0][] = ['text' => __('HSN'), 'feed' => 290, 'align' => 'right'];
    
            $lines[0][] = ['text' => __('Qty'), 'feed' => 435, 'align' => 'right'];
    
            $lines[0][] = ['text' => __('Price'), 'feed' => 360, 'align' => 'right'];
    
            $lines[0][] = ['text' => __('Tax'), 'feed' => 495, 'align' => 'right'];
    
            $lines[0][] = ['text' => __('Subtotal'), 'feed' => 565, 'align' => 'right'];
    
            $lineBlock = ['lines' => $lines, 'height' => 5];
    
            $this->drawLineBlocks($page, [$lineBlock], ['table_header' => true]);
            $page->setFillColor(new \Zend_Pdf_Color_GrayScale(0));
            $this->y -= 20;
        }
    }
    

    Let me know if you need the content of other files required for a module to work or if anything is not working for you as expected.
    I did not try to change the text color yet, so you might need to find a solution for that by yourself.
    EDIT: custom header added