As opposed to a PDF scan which can definitely not be forged at all? ;)
Nothing less than a digital signature can prove the integrity of a digital document, and even that is worthless unless the corresponding public key has been publicly made available via a separate and trusted channel, which is unlikely.
Anything that can be used to prove a document's integrity can generally also be used to identify where it came from and how it was produced, which is why we generally don't see any effort to do this at all.
In fact, plenty of things that can't prove a document's integrity can also be used to identify its source, which is why this isn't done; you can't be sure that you've sanitized the document enough to protect the leaker.
Nothing less than a digital signature can prove the integrity of a digital document, and even that is worthless unless the corresponding public key has been publicly made available via a separate and trusted channel, which is unlikely.