A digital signature is like a paper-based signature – each giving you a method for adding a unique identifier to a document. Both provide you with an easy way to show a person signed a particular document at a particular time, but a digital signature goes further as it can ensure that the document content has not been tampered with after it was signed. In addition, the document version history can let recipients see when a document was signed and see when any changes were made to it. This revision history is encrypted and stored inside the PDF and can be viewed at anytime via the Signatures pane.
To digitally sign a PDF document, you must have a digital ID, and to verify other people’s signatures, you must first share your digital IDs and add each other to your Trusted Contacts list.
Most Digital Signature software are not free, but I will introduce you some free software developed in JAVA and DOTNET with the power of iText and iTextSharp.
- PDF Digital Signer, it is portable version developed by RubyPDF
- jPDF Tweak, java version, Open Source.
- JSignPdf, Java version, Open Source.
Want to develop your own? please visit How to sign a PDF using iText and iTextSharp.