PDF is one of the most brittle and unforgiving formats I've ever worked with, in the sense that if you introduce even one stray byte into the middle of a file, you stand a 99% chance of wrecking the reading frame for the whole file (kind of like a frameshift error in DNA giving rise to "nonsense proteins"), totally breaking the world.
A custom keystroke filter to emulate one of those Department-of-Motor-Vehicle sort of forms where you have a little square box for every letter of your name, and you can fit only one character to a box.
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.