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.

JSignPdf is a Java application which adds digital signatures to PDF documents. It can be used as a standalone application or as an Add-On in OpenOffice.org. JSignPdf is open-source software and can be freely used in both private and business sectors.
The main features are,
* several different keystore types (as provided by Java)
* adding digital signatures to PDFs
* setting certification level
* PDF encryption and setting rights
* adding signature icon to PDF (visible signature)
* comprehensive Swing GUI
* support for batch processing (controlled by command line arguments, without GUI)
for details, please visit here.

pdfsam is an open source tool (GPL license) designed to handle pdf files. It requiers a Java Virtual Machine 1.4.2 or higher and it’s released in 2 versions, basic and enhanced.
pdfsam basic:
A simple tool designed to split and merge pdf files. With it’s simple and intuitive interface you can:
* split your pdf documents (into chapters, single pages, etc.).
* merge many pdf documents or subsections of them.
* extract sections of your document into a single pdf document.
* mix alternate pages taken from two pdf documents in straight or reverse order into a single document.
* save and load your environment to automatize your recurrent jobs.
* manage pdfsam settings and set an environment to load at start up.
pdfsam enhanced:
This is the enhanced version of pdfsam. In this version you’ll find all the basic features plus:
* encrypt your pdf files (RC40 bits, RC128 bits, AES128 bits) and set permissions on them.
* add a pdf frontpage or an addendum (or both) to your pdf documents.
* decrypt pdf documents.
* extract attachments from your pdf documents.
* set viewer options to a document to tell the viewer application how should open the document.
for details, please visit here.