Add a password to a PDF with real AES-256 encryption. The file is encrypted on your own device by QPDF running as WebAssembly, so neither the document nor the password is ever sent anywhere. Free, no sign-up, no watermark.
A PDF password is part of the PDF format itself, not a wrapper around it. When you set an open password here, the contents of the document are encrypted with AES and a key derived from your password. A reader that does not have the password has nothing useful to look at, only encrypted streams. That is why an encrypted PDF cannot be previewed in a file manager or read by a search indexer until someone types the password in.
The encryption happens through QPDF, a long-standing open-source PDF library, compiled to WebAssembly so it runs inside your browser tab. Your PDF is read into memory, encrypted, and handed back as a download. No copy is sent to a server, which also means nothing to delete afterwards and no retention policy to trust.
The PDF format defines two different passwords and they are not equally strong. It is worth knowing which one you are relying on.
The open password, sometimes called the user password, is required before the document can be displayed at all. This is genuine encryption. Without the password the content is mathematically unavailable, and AES-256 has no practical shortcut. Its only real weakness is a guessable password, which is why the strength hint appears as you type.
The permissions password, sometimes called the owner password, is different. It sets flags saying printing or copying is not allowed. Readers are expected to honour those flags voluntarily. Well-behaved software does; plenty of other software does not, and stripping the flags is trivial for anyone who wants to. If your document genuinely must not be read by the wrong person, set an open password. Use permissions for signalling intent, not for defence.
AES-256 is the default and the right choice almost always. It is the strongest encryption the PDF specification offers and every current reader supports it. The one reason to pick AES-128 is an old reader, roughly Acrobat 8 or earlier, or an embedded viewer in older hardware, which will refuse to open an AES-256 file. If you are sending a document to an organisation that still runs ancient software, AES-128 is still strong and will open.
There is no password recovery. The password is never transmitted and never stored, so if you forget it the document is gone. Put it in a password manager before you close the tab.
Sending a password-protected PDF and its password in the same email defeats the purpose. Send the password by a different channel, such as a message or a phone call.
Encryption protects the file, not the copies. If the unprotected original is still sitting in your downloads folder or in a sent-mail attachment, that copy is still unprotected.
Finally, a PDF that is already password-protected cannot be re-encrypted here until its existing password is removed, because the tool cannot read a file it cannot open.
No. Everything runs in your browser. You can disconnect from the network after the page loads and the tool still works, which is the simplest way to confirm the claim yourself.
No. We never receive it. Nobody can recover it, which is the point of encryption.
Yes, on current mobile browsers. Because the whole file is held in memory, very large PDFs are more likely to run out of room on a phone than on a desktop.
No. The page content, fonts and layout are untouched. Only the file container changes.