Two kinds of protection, and only one is real
A PDF can carry two different things people call protection. The first is an open password: the document's contents are encrypted, and without the password there is nothing to read. That is enforced by cryptography and it holds.
The second is a set of permission flags — no printing, no copying, no editing. These are recorded in the file and respected by every mainstream reader, but they are a convention rather than a lock. A tool that chooses to ignore them can. Both are offered here, and the difference is stated plainly rather than blurred, because people make decisions about what to send based on it.
Why doing this in the browser matters
Consider what uploading to a typical online tool involves: you send the confidential document and the password meant to protect it to a third party, over the network, to be processed on hardware you know nothing about. The threat model collapses.
Here, both stay on your machine. The file is read into memory, encrypted in the tab, and handed straight back as a download. There is no server to trust because there is no server involved.
Choosing a password
The cipher is not the weak point — the password is. A short or obvious one can be attacked offline at enormous speed, because the attacker has the file and can try as often as they like without anything rate-limiting them. Use a long passphrase, and send it to the recipient through a different channel from the document itself. Emailing both together protects nothing at all.
There is no recovery
Worth repeating because it catches people out: forget the password and the file is unrecoverable. No service, including this one, can open it for you. Save it in a password manager before you close the tab.