How to sign a PDF using iText and iTextSharp
If you are a Java or .NET developer, all the following articles are useful for you.
How to sign a PDF using iText and iTextSharp
if you just want a free Digitally sign PDF tool, try the following ones:
PortableSigner-Tool to sign Portable Document Files (PDF)
E-signing PDF documents with iTextSharp
Sign PDF online, but do not know how [...]