How to protect an already existing PDF with a password?

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How do I set a password for an existing PDF?

Posted on StackOverflow on Dec 2, 2014 by Sreekanth P

It's as simple as this, from EncryptionPdf:

Java

public void encryptPdf(String src, String dest) throws IOException {
    PdfWriter writer = new PdfWriter(dest, new WriterProperties()
        .setStandardEncryption(USER, OWNER,
            EncryptionConstants.ALLOW_PRINTING,
            EncryptionConstants.ENCRYPTION_AES_128 | EncryptionConstants.DO_NOT_ENCRYPT_METADATA));
    PdfDocument pdfDoc = new PdfDocument(new PdfReader(src), writer);
    pdfDoc.close();
    writer.close();
}

Note that USER and OWNER are of type byte[]:

Java

public static byte[] USER = "Hello".getBytes();
public static byte[] OWNER = "World".getBytes();

You have different options for the permissions (look for EncryptionConstants starting with ALLOW_) and also you can chose from different encryption algorithms.

Click How to protect an already existing PDF with a password? if you want to see how to answer this question in iText 5.