Why is my form being flattened without calling the flattenFields method?

Question

I am testing my method with this form https://help.adobe.com/en_US/Acrobat/9.0/Samples/interactiveform_enabled.pdf

It is being called like so:

Pdf.editForm("./src/main/resources/pdfs/interactiveform_enabled.pdf", "./src/main/resources/pdfs/FILLEDOUT.pdf"));

where Pdf is just a worker class and editForm is a static method.

The editForm method looks like this:

public static int editForm(String inputPath, String outputPath) {
    try {
        PdfDocument pdf = new PdfDocument(new PdfReader(inputPath), new PdfWriter(outputPath));
        PdfAcroForm form = PdfAcroForm.getAcroForm(pdf, true);
        Map<String, PdfFormField> m = form.getFormFields();
        for (String s : m.keySet()) {
            if (s.equals("Name_First")) {
                m.get(s).setValue("Tristan");
            }
            if (s.equals("BACHELORS DEGREE")) {
                m.get(s).setValue("Off"); // On or Off
            }
            if (s.equals("Sex")) {
                m.get(s).setValue("FEMALE");
            }
            System.out.println(s);
        }
        pdf.close();
        logger.info("Completed");
    } catch (IOException e) {
        logger.error("Unable to fill form " + outputPath + "\n\t" + e);
        return 1;
    }

    return 0;
}

Unfortunately the FILLEDOUT.pdf file is no longer a form after calling this method. Am I doing something wrong?

I was using this resource for guidance. Notice how I am not calling the form.flattenFields(). If I do call that method however, I get an error of java.lang.IllegalArgumentException.

Thank you for your time.

Answer 1

Your form is Reader-enabled, i.e. it contains a usage rights digital signature by a key and certificate issued by Adobe to indicate to a regular Adobe Reader that it shall activate a number of additional features when operating on that very PDF.

If you stamp the file as in your original code, the existing PDF objects will get re-arranged and slightly changed. This breaks the usage rights signature, and Adobe Reader, recognizing that, disclaims "The document has been changed since it was created and use of extended features is no longer available."

If you stamp the file in append mode, though, the changes are appended to the PDF as an incremental update. Thus, the signature still correctly signs its original byte range and Adobe Reader does not complain.

To activate append mode, use StampingProperties when you create your PdfDocument:

PdfDocument pdf = new PdfDocument(new PdfReader(inputPath), new PdfWriter(outputPath), new StampingProperties().useAppendMode());

(Tested with iText 7.1.1-SNAPSHOT and Adobe Acrobat Reader DC version 2018.009.20050)

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By the way, Adobe Reader does not merely check the signature, it also tries to determine whether the changes in the incremental update don't go beyond the scope of the additional features activated by the usage rights signature.

Otherwise you could simply take a small Reader-enabled PDF and in append mode replace all existing pages by your own content of choice. This of course is not in Adobe's interest...