Maybe I'm overlooking something here, but I don't know why leakers don't just use a forever stamp and drop something in the mail. Securing electronic communications seems freakishly hard by comparison. Is there some reason that is an obviously bad idea?
Using the postal service might be lower-risk, but it's not risk free.
If you try that, don't forget about the Mail Covers [1] program.
If you're mailing a reporter at the NY Times, you're at risk if you use your own handwriting. You might also be at risk if you use a printed label [2].
There's also the risk that your mail will be intercepted, and I wouldn't be too shocked to discover that government agencies were selectively (or not-so-selectively) reading our mail [3].
Its really sad how low trust in the rule of law has become. US Mail used to be sacrosaint. Damaging a mailbox is a felony to give an idea of how strong the law is in this area.
There's a significant amount of physical evidence from that. You'd have to make sure it's clean of any fingerprints and DNA (hair) for one. Plus printers will typically inject watermarks into the document. Handwriting is definitely a thing to be analyzed. And the location which you mail stuff from leaks another few bits of information.
It's probably a better idea than email, but not by all that much.
