It's accessible via a website so that you don't have to download anything, and everything running in your browser means the PDF never leaves your device, so it's 100% private.

Not having to download anything is not a plus. Much better to make an actual application that you download, and which never touches someone else's server.

This tool is hosted on a website, and you access that website like all other websites, using a browser interfacing with HTTP.

The functionality isn’t actually in the browser, it’s in the files downloaded via HTTP.

Yes, that's a good way of framing it. Though technically the files are executed in the browser. Will be releasing downloadable desktop version soon.