Once they have a copy, it's theirs and beyond your control. All you can do in the original pdf is to require a password to open it for viewing but not editing, and otherwise make it unchangeable (read-only) and unselectable (so they can't copy-n-paste) once the password's been given. They can save it to whatever filename they want, but they won't be able to change the password requirement or other restrictive attributes.
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