Open the PDF in Acrobat, and do one of the following:

• Choose Tools > Protect > More Options > Encrypt with Password.
• Choose File > Protect Using Password, and then choose Advanced Password Protection from More Options.

If you receive a prompt, click Yes to change the security.

Select Require A Password To Open The Document, then type the password in the corresponding field. Your password must be at least six characters long. For each keystroke, the password strength meter evaluates your password and indicates the password strength.

Select an Acrobat version from the Compatibility drop-down menu. Choose a version equal to or lower than the recipients’ version of Acrobat or Reader.

The Compatibility option you choose determines the type of encryption used. It is important to choose a version compatible with the recipient’s version of Acrobat or Reader. For example, Acrobat 7 cannot open a PDF encrypted for Acrobat X and later.

• Acrobat 6.0 And Later (PDF 1.5) encrypts the document using 128-bit RC4.
• Acrobat 7.0 And Later (PDF 1.6) encrypts the document using the AES encryption algorithm with a 128-bit key size.
• Acrobat X And Later (PDF 1.7) encrypts the document using 256-bit AES. To apply 256-bit AES encryption to documents created in Acrobat 8 and 9, select Acrobat X And Later.