My favorite book is a little known one called “Black Holes and Timewarps: Einstein’s Outrageous Legacy” and I reference it so much that I would really like a copy that’s fully mine and searchable, but I also care about the author receiving compensation for the purchase. Where do you get your PDF copies from?
Have you tried Archive.org or https://www.gutenberg.org/? And Calibre.
If you’re on MacOS, you can use Books to convert it or Preview specifically to pdf. These are both Apple stock apps.
There are plenty of resources online on how to convert to pdf. You can also check out Kobo, Smashwords, ebooks and this old article from https://www.makeuseof.com/tag/buy-ebooks-best-ebook-stores/). Have fun.