PDFs generation requires a lot of expensive compute (RAM, CPU). This is a free site with no ads or income and therefore (sorry) paying for more compute resources isn't something I'm willing to do. Therefore, server-side pdf creation is currently limited:
Fortunately, you can use do client-side pdf creation by printing the books in your web browser as PDF.
Instructions:
Example:
This is technically the same as server-side PDF creation but requires a few more clicks from users and a capable web browser (Google Chrome is the reference here but expect Edge and Safari to also just work)