jPDFEditor supports both adding a digital signature field to a PDF or applying digital signatures to existing fields. Digital Signatures are a way of authenticating the contents of a document. A document with a valid signature is guaranteed not to have been changed since being signed.
Create Digital Signature Fields : Create new unsigned digital signature fields
Modifying Digital Signature Fields : Modify existing field including clearing, moving, and properties
Applying Digital Signatures : How to apply digital signatures to a document
Validate Digital Signatures : How to validate a digital signature