Some documents — tax records, contracts, medical results, financial statements — shouldn't be openable by just anyone who gets hold of the file. Adding a password means that even if the PDF ends up in the wrong inbox or a shared drive, it can't be opened without the password you set.
Password protection genuinely can't be done with pure client-side JavaScript to a professional encryption standard, which is why this is one of the few tools on this site that uses a backend. The trade-off is worth it: proper AES-256 encryption instead of a weaker DIY implementation.
This tool requires the site's backend conversion service (see technical notes for site owner). It performs true server-side conversion rather than a client-side approximation, which is what makes the output reliable.