How to password protect a PDF in Adobe Acrobat Reader and shield your sensitive data

To password protect a PDF in Adobe Acrobat Reader, you'll need the paid Pro version, which starts at $14.99 a month.

How to password protect a PDF on a Mac computer to secure sensitive files

You can password protect a PDF on a Mac computer using the Preview app to secure files that contain sensitive information.
PC World

How to password protect a PDF before sending it by email

“Is there any way to password-protect and lock a sensitive PDF file before sending it as an attachment through email? I cannot afford Adobe Acrobat Pro and free primopdf didn’t work for me. Any help ...