I just Ctrl+P print the reader mode output to PDF. Takes 1 second. I've been doing it for years and it works perfectly. I have articles I saved from a decade ago in Dropbox. I'm honestly surprised people use a whole app for such a trivial feature, but yeah it's fair to have special needs. For me, Firefox reader mode is all I need, and you can just save it as PDF.

PDFs are the opposite of what I personally want from an archival format. I primarily want to be able to read the article on any screen size (on the go, when there's no Internet connection), and PDF falls flat for that.

Even on my computer, not many pages have a nice print stylesheet, and the result frequently looks horrible/nothing like the original page.