My guess is that my brain has subconsciously tuned out engaging pdf content because of how difficult it is to use in-browser... Especially when dealing with text sizes and zooming sigh. It's even worse with pdfs on mobile :(
Also the sudden break from "website" to "pdf" format is often jarring.
PDF is a beast. It's a ridiculous file format. There's a reason why even after all these years, reading PDFs still kinda sucks.
On android I have to use a firefox fork called iceraven to be able to install pdf.js extension to use mozilla's own pdf.js to load pdf's in my tabs. Afaik, there's no other way to do it.