Password-protecting a PDF: what actually keeps it safe
User vs owner passwords, what AES-256 encryption protects, why "restrictions" are weak, and how to choose and share a PDF password safely.
By Fileixa editorial team, Creates Online LLC · Published · 5 min read
Two kinds of PDF password
An open password (the "user password") encrypts the document: without it the file cannot be opened or read. A permissions password (the "owner password") lets the file open for anyone but asks reader software to block printing, copying or editing.
Only the first is real security. Permission restrictions depend on the reading software choosing to honour them, and many tools ignore them. If the content is confidential, it needs an open password.
What encryption protects and what it does not
With AES-256, the strongest standard encryption for PDF, the file's contents are scrambled so that without the password they are unreadable, even to someone who copies the file. Current Adobe, Apple, Google and Microsoft readers all support it. Older PDFs protected with the legacy RC4 method are much weaker, which is one reason to re-protect old files.
Encryption does not stop someone who knows the password from forwarding the opened document, taking screenshots or printing it. It protects the file in transit and at rest, not what an authorised reader does with it.
Choosing the password
Encryption is only as strong as the password. Cracking tools can try billions of guesses, so short or common passwords fall quickly. Use at least 12 characters, or a passphrase of four or more random words, and avoid names, dates of birth and anything printed in the document itself.
Sharing it safely
The most common mistake is sending the password in the same email as the file.
- Send the file by email and the password by text message or phone call, or the other way round.
- Agree on a password in advance with people you send documents to regularly.
- Keep your own unencrypted copy. A forgotten AES-256 password cannot be recovered by anyone.