Put a password on your PDF with real AES-256 encryption — without the password the file will not open. Optionally block printing, copying or editing, also without a password. The file stays in your browser.
You can also do without a password: tick at least one restriction. The file then opens for everyone and only the restrictions apply.
Keep the password safe: without it the file truly cannot be opened — there is no back door.
This tool runs entirely in your browser: your PDF is not sent to a server. A PDF with a password opens here with that password, or you remove the protection with the Unlock tool; the password also stays in your browser.