Why would you want functionality like this to be in a browser?
Isn't a browser for interfacing to http?
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.
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.