Likely part of a web scrapping project that may require navigation or other browser things to get to. Or they are trying to avoid detection with a more real browser (you have to change some stuff first).

Can you recommend any resources for getting started with pdfium?

Sorry I'm in that position myself. A couple of years ago I compiled it without too much difficulty, and the included example I tried out rendered a page to a .png. Doing that for that for thousands of PDFs would just take a loop (or find -exec) if you have the files locally.