There are free programs available to create digital signatures with certificates on PDF documents:
- "Portable Signer" (GUI + commandline for Linux, OSX, Windows):
http://portablesigner.sourceforge.net
- "CAcert PDF Signer" (commandline only for Linux, OSX, Windows):
http://www2.futureware.at/svn/sourcerer/CAcert/PdfSigner/ Nautilus Script for PdfSigner: http://www.cacert.at/svn/sourcerer/CAcert/PdfSigner/pdfsigner-nautilus.sh
OpenSignPDF: http://opensignature.sourceforge.net/english.php : supports Smartcards and Timestamping
"Footprint" (PDF Writer & Signer, Java 1.6+ library): https://footprint.dev.java.net/
Adobe´s documentation on PDF Security: http://blogs.adobe.com/security/2007/04/acrobat_and_reader_security_do.html
A new software, which I haven´t tested yet: http://www.cabaret-solutions.com/de/downloads/linux
OpenOffice feature request with voting: http://qa.openoffice.org/issues/show_bug.cgi?id=47895