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> Of course this is completely false safety given that many programs can edit PDFs.

Well, unless you sign the PDF. Even more, you can sign each edit separately, so you can do things like add content and signatures and still verify who added what. Meaning: one party can create PDF with forms, sign it, then the party filling out the form can sign their own changes for authentication.

And let's not forget the fact that PDF renders correctly on pretty much any machine you put it on - this is incredibly important.

> Of course this is completely false safety given that many programs can edit PDFs.

But can you edit a properly signed[1] PDF without breaking it? From an integrity perspective, that's what matters; otherwise, it's just inherently more portable until non-repudiation becomes relevant.

[1] As in not SHA-1: https://shattered.io/

Including Word nowadays!