Documents are forgeable, but we generally believe documentary evidence. (Well, in courts we need a person to vouch for a document, but we already also need that for video too-it's a very testimonial system.)

When two people dispute authenticity (outside of courts), we reflexively gauge the credibility and perceived motives of those involved.

It's a rough system, prone to error, but my point is we navigate these things socially without assuming bad faith for all documents.

It's like how home locks offer close to zero protection, but they are still a normal part of our world.

We can't be sure a document is real, can't be sure a lock will matter, but we are all busy, so we shrug and go on with our day in most cases.