SE makes ebooks available in four formats: "compatible" epub; "advanced" epub; kobo-compatible ("kepub"); and kindle ("azw3"). No PDFs.

One of the SE editors experimented with turning SE ebooks into PDFs, though. See more about that here: https://groups.google.com/g/standardebooks/c/Xy2bwiexLeM/m/f...

I don't know their reasons but PDF is a rather problematic format so I suspect that's why.

You can run their EPUB through Pandoc to convert yourself, or put some effort in and setup your own Calibre instance which will do something similar when you ask it to.