Modifying digitally signed PDF's

Question

Hi all,

I'm looking for a way to add a watermark/text overlay to a PDF document that contains a digital signature. Currently we've licensed PDFlib which does a great job of handling the overlays, but the problem is the current process will lose/wipe-out a digital signature added using Adobe's PDF creator software.

I'm aware that modifying the document after it's been signed will partially invalidate the signature (the post-signature modifications won't be signed, but the document as-signed is still available to view)

Thanks,

Dave

Answer 1

Do you mean that adding a watermark will completely remove an existing digital signature? If so then that sounds like a PDFlib bug. You're absolutely right that modifying the document in any way after it's been signed will invalidate the signature, but the signature should still be present and visible. It shouldn't just vanish. I'd try reporting it to PDFlib to be fixed before anything else.

If you can't get any resolution that way, then you can find other APIs to use, but of course you'd have to either learn a new API and/or license a different tool. We offer one called EasyPDF SDK that supports digital signatures as well as watermarks (http://www.pdfonline.com/easypdf/epsdk%5Fmanual/index.htm?page=reference%2Fpdfprocessorsdk%2Fpdfprocessor%2Fm%5Faddwatermark.htm) if you decide to look for something else. There's a trial copy available at www.pdfonline.com.

Cheers

Answer 2

Have you looked at Itext/ISharp?