Protect PDF

Password-Protect a PDF with AES-256

This applies real encryption. The document's contents are encrypted with a key derived from your password, and every reader — Acrobat, Preview, Chrome, a phone — will demand that password before showing a single page. Without it, the file is noise.

That is worth stressing because plenty of "PDF protection" features are not this. Setting a read-only flag or stamping a confidentiality notice does nothing to a determined reader. Encryption does.

Before you type a password

Write it down first, in whatever you use to store passwords. Then type it here.

There is no recovery, no reset link, and no support address that can help. The password is used in your browser to derive an encryption key, and it is never stored, transmitted or recorded anywhere. If you forget it, the document is gone. Not "gone until you contact us" — gone.

This catches people out most often with files they encrypt and then archive. You will remember the password this week. Next March, you will not.

The minimum length accepted is four characters, which is a floor rather than a recommendation. Four characters is trivially brute-forced. For anything you genuinely care about, use a long random string from a password manager, or a passphrase of several unrelated words.

What the encryption covers

Everything: page content, text, images, embedded files and metadata. This is the AES-256 scheme from PDF 2.0, the current standard, and it is what corporate and legal workflows expect.

What it does not include is the granular permission system some PDF tools expose — separate switches for printing, copying text or adding annotations. Those flags are not offered here, and it is worth understanding why they are a weak control anyway: they are instructions to the reader software, not enforcement. A reader that chooses to ignore "do not print" will print. An open password is the part with actual cryptography behind it, and that is the part this tool does.

What a password does not protect you from

Encryption controls who can open the file. It has nothing to say about what happens next.

• Anyone who opens it can screenshot it, print it, or retype it.
• Anyone who has the password can pass on both the file and the password.
• Emailing the encrypted file and the password in the same message protects nothing at all. Send the password by a different channel — a phone call, a message app — and never in the covering email.
• If someone had a copy before you encrypted it, they still have that copy.

Encryption is the right tool for "this attachment might be forwarded to the wrong person" and the wrong tool for "I need to control what the recipient does with it".

Order of operations

Encrypt last. Once a file is encrypted, the other tools here cannot read it — merging, splitting, rotating and watermarking all refuse encrypted input at upload, and so will most other software.

So do the document work first, check the result, then protect it. If you already encrypted something and need to change it, Unlock PDF will decrypt it with the password so you can edit and re-protect.

Practical notes: the file is encrypted in your browser tab and downloaded from there, so the password never travels. Inputs up to 100 MB are accepted. If you want a visible ownership marker as well as encryption, apply Add Watermark before you encrypt, not after.