How to password protect a PDF

How to password protect a PDF, step by step

Password protecting a PDF takes under a minute and needs nothing to install:

1. Open the Password protect a PDF tool and drop the file into the window, or click to choose it. It stays in your browser the whole time.
2. Write the password: at least 3 characters. The longer and less obvious it is, the stronger the protection.
3. Download the protected PDF, already asking for the password whenever anyone opens it.

The processing runs on your computer, not on a server: the file never leaves it during the process.

Which cipher protects the file? AES-128

The tool encrypts with AES of 128 bits, applied to the whole file after the PDF header is forced to version 1.7 right before encrypting. It is not the newer 256-bit AES: the library this tool uses cannot yet generate that stronger encryption level.

AES is a robust cipher used as an industry standard, and the strength of the protection comes mostly from the password, not the algorithm. The 3-character minimum is the technical minimum, not a recommendation: a long, unpredictable passphrase protects far more than a short password that is only technically valid.

And there is no back door: because the encryption is real, there is no recovery for a lost password. BestPDFInTheWorld does not keep a copy of the password, and there is no way to recover it once the PDF is protected.

Opening password and permissions: not the same thing

The PDF format knows two different protections: the opening password, which asks for the key before you can even open the document, and the permission passwords, which restrict what can be done with it, printing, copying or editing. This tool applies the opening password: anyone without it cannot open the file. High-resolution printing, however, remains allowed to whoever has it, and there is currently no separate option to restrict printing, copying or editing.

If you want to understand the differences between the two passwords and the ciphers of the format, read the guide PDF passwords: what actually protects.

Tips for password protecting PDFs

• Keep the password safe before downloading. There is no recovery: if you forget it or lose it, nobody, including BestPDFInTheWorld, can open the file again. Choose something you will remember, confirm it, and if in doubt save it in a password manager.
• An already protected PDF will not open here. If the input file already has a password, the tool shows a generic error as soon as you load it. To change the protection, first open the PDF with the current password and save a copy with no protection at all, to remove the encryption from one of your files, use the Unlock PDF tool.
• Heavy file to send? Follow up with the Compress PDF tool to reduce its size.
• It works on your phone. You can choose the file, write the password, and download the protected PDF directly in your phone's browser.

Is password protecting PDFs free? What are the limits?

No. This tool is part of the Premium subscription, EUR 1.99 a month, which removes the limits on every tool and adds OCR. Sign PDF opens on the free plan, with one signature a day.

Sources

• BestPDFInTheWorld, Proteger PDF com palavra-passe
• MDN, File API
• pdf-lib