Today, I just finished a bid(project) from RAC, that is “Digital Signature and Timestamp to pdf document with iTextSharp”.
the requirement is:

PDF Sign and TimeStamp

Provide a fully functional desktop application and source code that can:
Sign a PDF file from user certificate (.pfx)
Add timestamp (RFC 3161) from designated TSA
Save signed pdf in designated file
File can be verified with Adobe Acrobat Reader
Support for multiple signatures

Written in C# using open source libraries Itextsharp and Bouncycastle

App similar to http://www.codeproject.com/useritems/Esignature.asp
but with timestamp capability

Similar JAVA apps available as open source but prefer native C# app.

reference:

http://security.polito.it/ts/

http://www.codeproject.com/useritems/Esignature.asp