How to Lock, Protect or Encrypt PDF Files

Here, I have the best available manual method to encrypt a PDF file using Adobe. The steps are as follows:

1. Select Tools > Protect > Encrypt > Encrypt with Password after opening the PDF.
2. To modify the security, select Yes when prompted.
3. After choosing to require a password to open the document, enter it in the corresponding field. The password strength meter calculates the strength of your password for each keystroke and displays it.
4. From the drop-down selection under Compatibility, choose an Acrobat version. Select an Acrobat or Reader version that is equivalent to or less advanced than the recipients' version.

This manual way work properly but is not a good option for a user from a non-tech background. Thus, you can try a much better professional approach using PDF Encryption Tool. It comes with free de4mo to evaluate the tool.