A password-protected PDF is not processed in the browser, even one that only restricts printing or copying. If you know the password, open the file in your PDF app and save it without protection, or use the desktop version, which asks for the password and keeps the encryption. If the password is lost,
PDF Password Recovery can try to get it back. A signed PDF is still cleaned, but a warning appears above the results: the signature stops validating, so keep the signed original if you need it.