PDF password encryption is included with Adobe Acrobat and other PDF creator software. A password is used to encrypt the PDF file. The same password is used to decrypt it.
You can choose the encryption algorithm and strength (key length) in Adobe Acrobat to support backwards compatibility with older versions. However, AES 256 bit is the most secure.
Encrypting PDF files with a password is not recommended if you want to protect PDF files from unauthorized use and distribution because:
- Passwords can be shared – they have to be sent in readable form so users can copy and forward them
- The password interface has to be exposed so can be readily attacked by password recovery programs – see Removing PDF Passwords
- Strong passwords are difficult to set up and use, so weak passwords are normally used
- If you use different passwords for each PDF document then you have to manage lists of passwords – this can become complicated
- Regardless of how strong the Permissions password used (the password used to restrict PDF document use) it can be easily removed – see Removing PDF Passwords – and therefore provides NO protection
- Once a user has the password they can unprotect the PDF and do what they like with it unless DRM controls have also been used