Which encryption is used?

AES-128 encryption by default, with AES-256 available for stronger protection.

What happens if I lose the password?

There is no recovery. Keep your password in a safe place such as a password manager.

Yes by default. You can also restrict printing and copying when setting the password.

No. The file is encrypted in memory and removed once you download it.

Add a password to your PDF so only authorized users can open it