Adding a digital signature to a pdf file [duplicate]

Possible Duplicate:
Digitally sign PDF files

In the application a user will upload a PDF file and then upload a personal signature created using a digital pen. How can I embed this signature in the pdf file?

Can a PDF be digitally signed twice?

You cannot use multiple digital signatures, since later signatures will invalidate the previous signatures.

Can you add a signature to an already signed PDF?

If you received a PDF digitally signed by others, you can sign it, but can't edit it. While signing a PDF, if the signer chooses to lock the document after signing, the document becomes read-only for everyone, including the signer.

Use a PDF editing library. I've used this one before: http://pdfsharp.com/PDFsharp/