Protect PDF — Lock a PDF with an AES-256 Password

Lock a PDF with a password using genuine AES-256 encryption, applied by the open-source QPDF engine right in your browser — free, with nothing uploaded.

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Protect PDF — Lock a PDF with an AES-256 Password guide

Password-protect a PDF in your browser with real AES-256 encryption — the same protection professional tools apply — powered by the open-source QPDF engine compiled to run entirely on your device. Set a password, download the locked copy.

How to use this tool

Choose your PDF with the file button or drag it onto the panel. The tool reads it locally and reports the page count.
Type the open password the reader will need. Type it a second time in the confirm box — a typo here locks everyone out, including you, so the two entries must match before the button wakes up.
Optionally set a different owner password. The open (user) password is what a reader types to view the file; the owner password controls editing and permission changes in full PDF editors. Leave it blank to use the same password for both.
Click Protect PDF and save the locked copy. Test it once: open the download and confirm it asks for the password before sharing it anywhere.

A worked example

Before emailing a spreadsheet export of salaries as a PDF, an HR assistant protects it with a password shared by phone. If the email is forwarded or the attachment is left in a shared inbox, the file itself still refuses to open without the password — the protection travels with the document instead of depending on where it is stored.

What you can do with this tool

Password-protect a PDF with real AES-256 encryption powered by the open-source QPDF engine running entirely on your device.
Type the open password a reader will need to view the file.
Confirm the password in a second box — the two entries must match.
Set a different owner password to control editing and permission changes, or leave it blank to use the same password for both.
Download the locked copy.
Test the download once to confirm it asks for the password before sharing it.

How it works

Encryption is done by QPDF — the respected open-source PDF toolkit — compiled to WebAssembly and run in your browser. It re-encrypts the file with the PDF standard’s AES-256 scheme (the strongest the format defines), so any standard reader — Acrobat, Preview, Chrome, Edge — will demand the password you set. Your file and your password never leave your device; there is no server copy of either.

Frequently asked questions

How strong is the encryption?

The file is encrypted with AES-256, the strongest scheme the PDF standard defines and the same one professional tools use. Real-world strength then depends on your password: a long random passphrase is effectively unbreakable; a short or common password is not, whatever the algorithm.

What is the difference between the user and owner passwords?

The user (open) password is required just to view the document. The owner password unlocks full control — changing security settings and permissions in an editor. Many people set only one; setting both lets you share the viewing password widely while keeping control yourself.

I forgot the password. Can you recover the file?

No — and neither can we open it for you. The encryption is genuine, the password was never sent to us, and there is no master key. Keep passwords in a password manager, especially for archives.

Will the locked file open everywhere?

Yes. AES-256 PDF encryption is part of the PDF standard, so Acrobat, Apple Preview, browsers and phone readers all prompt for the password and open it correctly. Very old readers (pre-2017) may not support the newest revision — test with your recipient’s software for critical sends.

Is my file or password uploaded?

Never. QPDF runs as WebAssembly in your browser tab; the document and the password exist only in your device’s memory and are gone when you close the tab.