How to protect an already existing PDF with a password?

Question

How to set password for an existing PDF?

Answer 1

Did you look at the EncryptionPdf example in chapter 12 of my book?

It's as simple as this:

public void encryptPdf(String src, String dest) throws IOException, DocumentException {
    PdfReader reader = new PdfReader(src);
    PdfStamper stamper = new PdfStamper(reader, new FileOutputStream(dest));
    stamper.setEncryption(USER, OWNER,
        PdfWriter.ALLOW_PRINTING, PdfWriter.ENCRYPTION_AES_128 | PdfWriter.DO_NOT_ENCRYPT_METADATA);
    stamper.close();
    reader.close();
}

Note that USER and OWNER are of type byte[]. You have different options for the permissions (look for constants starting with ALLOW_) and you can choose from different encryption algorithms.

As for the parameters: src is the path to the existing PDF. dest is the path of the encrypted PDF. It should be obvious that you can not write to a file while you are reading it. That is explained here: How to update a PDF without creating a new PDF?