Unless you spend a lot of money on a DRM solution there's no real way to do it. A person will always be able to send someone else the file with their password. Also, some PDF viewers don't enforce these security policies at all, and allow files to be opened even without the password, or will remove the policy entirely. That is more tricky if it's a file-open password, but not impossible.
One possible workaround is to embed the person's name in the file (as a watermark, for example). That might cause them to think twice before sharing it, as it would be clear who did it and they'll be open to legal action against them.
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