AES-256 is the modern encryption choice for compatible PDF readers, but the encryption is only as useful as the password and the way it is shared. A short reused password can undermine strong cryptography.
PDF permissions can discourage printing or copying, but they are not a substitute for access control or digital rights management.
Choose the right password
Use a long, unique passphrase that is not based on public information. Save it in a password manager. If the recipient needs the password, send it through a different channel from the PDF—for example, send the file by email and the password by an agreed messaging service.
Protect and verify
Open Protect PDF, choose AES-256 where offered, enter the password carefully, and process the file. Download it, close your current viewer, then reopen the saved copy to confirm that a password is requested.
- Test the correct password.
- Test an incorrect password.
- Keep an unencrypted archival copy in secure storage if your workflow requires it.
Compatibility and recovery
Very old PDF readers may not support modern encryption. Test the recipient’s required software before deleting any original. A properly encrypted file cannot be recovered by the service if you forget its password.