Sunil
Here is some information...
How digital signature works
http://www.adobe.com/devnet/acrobat/
Also, see attached socs.
How a pdf can be signed using workbench
Workbench is the development tool for creating LiveCycle ES2 processes. To sign a PDF on the server side (in an automated fashion) you need to have LiveCycle Digital Signatures ES2
For more info...
http://www.adobe.com/products/livecycle/
http://www.adobe.com/products/livecycle/digitalsignatures/
How to sign entire pdf
When a digital signature is applied to a PDF, the entire PDF is "signed". You cannot sign specific pages or sections.
How to sign/block only few fields in the pdf
You can specify "field collections" when creating a PDF with Adobe Designer. You then associate the field collection with a particular signature field. When the signature field is signed, the associated field can\will be locked.
Hope this helps
Steve