Lock PDF Online Free
Add a password to a PDF so only people who know it can open the file. It is encrypted with AES-256 right here in your browser, so your document and password are never uploaded.
About this tool
How to password protect a PDF
- Upload the PDF you want to secure.
- Type a password of at least four characters, though longer is meaningfully safer.
- Press the button and download the protected file.
From then on, anyone opening the document, in any reader on any device, is asked for that password first.
How the encryption works
This tool runs QPDF, a long-established PDF library, compiled to WebAssembly so it executes inside your browser tab instead of on a server. Your file and your password are handled entirely on your own device; neither is transmitted anywhere. That matters more here than on any other tool on this site, because sending a confidential document and its password to a stranger's server would defeat the point of encrypting it.
The document is encrypted with AES-256, the same algorithm used to protect classified material and banking traffic. The password you choose is set as both the user password (needed to open the file) and the owner password (needed to change its permissions).
Choosing a password worth using
AES-256 is not the weak point. The password is. A short dictionary word can be brute-forced offline by anyone who has the file, because they can test guesses as fast as their hardware allows, with no lockout to stop them. A passphrase of several unrelated words is both easier to remember and dramatically harder to break.
Send the password through a different channel than the document itself. Emailing both together protects nothing.
Frequently asked questions
Can the password be recovered if I forget it?
No, and that is the point of real encryption. There is no master key and no reset. Store it somewhere safe before you send the file on.
Does the file look or print any differently?
No. Pages, fonts and images are unchanged; only access to them is restricted.
Will it open on phones?
Yes. AES-256 PDF encryption is supported by every current reader on desktop and mobile.
Can I remove the password later?
Yes. Use our Unlock PDF tool, with the password.
Written and maintained by Nudge Systems, Surat
Last reviewed