What Would Be Some Good Approaches To Writing A Browser Extension?

What would be some good approaches to writing a browser extension that allowed the browser to view *only* HTML pages that contain links to a particular type of file (PDF)?

To me all are good browsers. Very much the same. I think each platform has it’s “major” browser. Chrome for Chromebooks, Safari for Mac, and Edge now for the later Windows. So there has to be a reason for me to just add the bulk of another browser onto my hopefully lean and mean main gal. Currently I’m trying out and really liking Opera GX browser in it’s night or dark mode. Designed for my gaming machine Dell’s Alienware 13r. It’s proven great on the internet. It’s fast i7 Intel memory chip and Get Force screen processing. Beautiful. For some probably bizarre reason I got a notice from Apple no to use the Chrome browser with a Mac. But use Safari. I complied. I was using Echo browser with Windows before Opera GX. Supposed to render me less web trash and I think that is so. I haven’t seen a reason to replace Chrome on the Chromebooks so staying with Chrome there. I do really like the whole web/internet idea of Chromebook apps and web storage.