PDF passwords and restrictions are not the same thing
One stops the file opening. The other only asks software politely not to print. Knowing which you have changes what to do.
A PDF can carry two different kinds of protection, and they behave nothing alike. An opening password, sometimes called a user password, means the file will not display at all until the password is typed. Without it there is nothing to see, and no tool can read, convert or compress the document either.
The other kind is an owner password, which sets permissions. The file opens for anybody who has it, and the restriction only says that printing, copying or editing is not allowed. Every mainstream PDF reader honours that request, but the content was never hidden: it was on screen the whole time. This is why a restricted PDF can be unlocked without knowing any password, while a genuinely encrypted one cannot.
That distinction decides what to do with a file you have been sent. If it asks for a password to open, you need the password, and the sender is the only source. If it opens but will not print, you can remove the restriction yourself, and Unlock PDF does it with the password box left empty.
What neither kind does is control the document after it is open. Anyone who can read it can screenshot it, retype it or forward it. If something in the file genuinely must not travel, a password is the wrong tool. Take the sensitive part out of the document instead.
Tools for this