Password-protect a PDF — without uploading

Encrypting a document means, by definition, that you do not want other people reading it. Uploading it to a website to achieve that — along with the password — hands both halves to a third party in the one operation where that is least acceptable. The file is unprotected at the moment it travels, and the password travels with it.

Both stay here. The file and the password are processed in your browser by Ghostscript compiled to WebAssembly; nothing is transmitted, because this site has no backend and a Content-Security-Policy blocks third-party requests. You can disconnect from the network before typing the password and still get an encrypted file out.

PDF encryption has two distinct passwords, and the difference is worth understanding because it determines what protection you actually get.

• The open (user) password is required to view the document at all. Without it the file cannot be decrypted, so the content is genuinely inaccessible. This is real protection.
• The owner password separately controls permissions — printing, copying, editing — on a file that can already be opened. This is much weaker: the restrictions are flags that compliant viewers choose to respect, and plenty of software ignores them entirely.

If you leave the owner password empty, the open password is used for both. For actually keeping a document private, the open password is the one that matters. Never rely on permission flags alone to stop someone copying text.

This uses 128-bit RC4, PDF encryption revision 3 — the strongest the WebAssembly engine supports. Being straightforward about what that means: it is compatible with essentially every PDF viewer ever made, and it will stop casual access comprehensively. Someone who receives the file and does not have the password cannot read it.

It is not modern AES-256, and RC4 is an old cipher with known weaknesses. For a document sent to a colleague, a statement stored on a shared drive, or paperwork emailed to an accountant, it is entirely adequate. For genuinely high-value secrets facing a determined, well-resourced attacker, use full-disk encryption or an encrypted archive with a modern cipher instead — and do not let any tool convince you that PDF encryption is a substitute.

As always, the encryption is only as good as the password. A short or reused one undermines any cipher; the password generator produces crypto-random passwords on your device, and there is a guide on generating a strong password.