Protect a PDF with a password
File › Protect with Password… (⌥⌘P) encrypts the PDF with AES-256. Add an owner password to stop printing, copying text or editing without it. File › Remove Password… takes protection off. Protecting is Pro; opening protected files is free.
Protecting a PDF is Pro. Opening a protected PDF is free, and Remember in Keychain saves you typing its password again.
Protect
Choose File › Protect with Password… (⌥⌘P). Type a Password to open and Confirm password; a hint shows when it is fair (8 characters or more) or strong (12 or more).
Also restrict adds an Owner password. Without it, you can stop Printing, Copying text and Editing and annotating.
Press Protect. The file is encrypted with AES-256 and saved as a new version: File › Revert To brings back the unprotected file.
Remove
Choose File › Remove Password… and confirm with Remove Password. If the file has restrictions, Better PDF asks for the owner password.