How to Password Protect a PDF

Some tools only set a flag that a PDF viewer chooses to respect — the file itself is unprotected, and other software can ignore the restriction entirely. Protect PDF applies standard AES encryption to the file content, so opening it without the correct password isn't possible regardless of which viewer or library is used.

Upload the PDF, set a password of at least 4 characters (longer and less guessable is better for anything genuinely sensitive), and run Protect PDF. The output is a new encrypted file — the original upload is untouched.

Send the password through a different channel than the file itself — a text message if the PDF went by email, for instance — so intercepting one doesn't hand over both. If you later need to remove the password from a file you control, Unlock PDF reverses this with the correct password.