Protect a PDF document with a password

What the password does

The protected PDF is encrypted with AES-256, the strongest encryption the PDF format offers. AES is the encryption standard published by the US National Institute of Standards and Technology as FIPS 197, and the encryption itself is done by the open-source qpdf library. Any PDF reader, such as the one built into your browser, Preview on a Mac, or Adobe Acrobat Reader, asks for the password before it shows a page. Without it the content cannot be read.

The same password also unlocks full rights, so whoever opens the file can print and copy from it. If you need to limit what a reader can do after opening, a password on a PDF is not the right tool: share the document as a view-only link instead.

Send the password separately

A password sent in the same email as the file protects nothing if that email is forwarded or read by the wrong person. Send the file by email and the password by text message or a phone call.

If you forget the password

There is no recovery. This tool does not keep a copy of your file or your password, and AES-256 cannot be guessed in any practical time. Keep the original, unprotected file.

How secure is a password protected PDF?

It depends on the encryption inside the file and on the password. Very old PDFs were protected with 40-bit RC4, which free software can break quickly. AES-256, which this tool uses, has no known practical attack. If a PDF was protected years ago, protect it again here to bring it up to AES-256.

A PDF can also carry two different passwords. The open password encrypts the content: nothing can be read without it, and it is the one this tool sets. A permissions password only asks the reader app to block printing, copying or editing. The pages can still be read, and many apps ignore the restriction, so it is not protection against someone determined.

Finally, the password itself. Encryption cannot be broken, but a weak password can be guessed. Four or more unrelated words are easy to type and hard to guess. A password used on another site is the weakest choice of all, because leaked passwords are the first ones tried.

Other ways to password protect a PDF doc

On a Mac, with Preview

1. Open the PDF in Preview and choose File, then Export.
2. Click Permissions, tick the option to require a password to open the document, and type the password.
3. Save. The exported copy asks for the password.

In Microsoft Word, when the document starts as a Word file

1. Choose File, then Save As (or Save a Copy), and pick PDF as the file type.
2. Click Options and tick Encrypt the document with a password.
3. Type the password twice and save.

In Adobe Acrobat (the paid version)

Open the PDF, choose the Protect tool, then the option to protect with a password, and set a password for opening the document. Acrobat Reader, the free version, cannot add a password.

A password on the file, or a password on a link

A password on the PDF stays with the file forever. That is useful for files that will be stored or passed around, but you cannot take it back, change it for one person, or tell whether anyone opened it. When you send a PDF to a client or a colleague, a PDF link with a password can do more: you can set it to expire, keep it view-only, delete it, and get an email when it is opened.