There are two different things people mean by protecting a PDF, and conflating them is how documents leak.
The first is controlling who can open the file at all. That is a password, and it works — the contents are encrypted, and without the password there is nothing readable to find. This tool does that, and it also does the reverse: a file you were able to open can come back out as a copy with the password removed.
The second is stopping a particular person from seeing a particular thing inside a document that they are allowed to open. A password does nothing for that. Once the file is open, everything in it is available — every name, every figure, every comment left in the margin. If a name has to be gone, it has to actually be deleted from the file, which is a different tool and one we keep deliberately separate.
The warning that matters most here is about loss. A PDF password has no recovery mechanism: it is not an account you can reset, there is no email link, and there is no copy anywhere because the encryption happens in your own browser. A document whose password is forgotten is a document nobody opens again. The panel says exactly that, in plain words, before it runs — because the moment to hear it is not afterwards.
There is more you can set than the open password. Under More options the panel offers a separate owner password and a set of permissions — printing at full or reduced quality or not at all, copying text, editing, annotating, filling forms, and reordering pages. Be straight about what those are: a PDF carries them as flags asking readers to behave a certain way, most software honours them, and software that ignores them is not doing anything clever. They are a request, not a lock, and the panel describes them as what they are.
Everything happens in this browser, including the encryption, so the password itself never travels. There is no signup, no limit, and no watermark.