Does anyone out there know if there is a practical way to sign PDFs to enable me at a later date to determine if a PDF has been tampered with?

I know you can password protect them etc but what I want to be able to do is create a PDF and then if needed at a later date get that PDF back from a client and verify that the signature is mine (or my companies), and that they aren't using a modified (or otherwise regenerated) PDF.

Preferably this could be done with existing GPG (PGP) Keys.