Digital Signatures - Word, Excel, Powepoint and Acrobat

Hi there, we are working on a quality control project for Word, Excel, Powerpoint and Adobe involving electronic or digital signatures. Couple of questions:

how do we create them?

How do we add our signature to a document as certification that we have signed off on its contents? Note that this is not just adding in a scan of our physical signature, but a system of certifying that we vouch for the document content - only the person who owns the signature should be able to add it to the document and the document should be locked down after the signature has been added.

Is there any way of automatically removing electronic signatures if the document is edited after sign-off?

Thanks

Susan