If you can buy the ebooks in a different format (either DRM-free or with a compatible plugin), you can use Calibre to convert them to PDFs.

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.

If you want a PDF for any book, try, at least, as much as you can, to get them directly from the author, if the author is an Indie Author they will willingly do it for you or, maybe, it is available directly on their own online store.

By doing so, you are bypassing all those companies taking, as they say "a small cut". So you are sure that your money goes directly to the author and no to the "I-take-a-small-cut company". That goes also for EPUB format etc 😀

I'd suggest contacting the publisher and see if they can work out a special deal for you.

You probably will need to be a bit persistent as the first person who you reach will probably be following a script to reject you.

a) Anywhere you buy that book from that isn't using DRM, isn't paying the author.

b) If they're using DRM you'll have to use some sort of proprietary software to use the book (i.e. not a straight PDF you can open with any PDF reader)

c) Owning the physical copy doesn't give you any more right to scan the book than it does downloading it DRM-free or bypassing DRM. Depends on the country, but making your own copy is a grey area. (so if you're going to scan it or crack it, just download it from the internet archive or something - otherwise you're just wasting time, money, and resources and contributing unnecessarily to the heat death of the planet/universe.)

VitalSource provides lifetime digital licenses but requires their proprietary software (Bookshelf) which is available on a lot of different devices. They work with publishers to pay royalties which should go to authors. I think they have a return policy if you try it out and it isn't working for you. (I've never used it myself)

It's also on Kindle 🤮 and Kobo and a bunch of other places. The publisher probably provides links to sites where you can get it.

You can also check your library for a digital copy and take it out whenever you want to search it, I suppose.

Smashwords offers their ebooks in a variety of formats, including pdf.

Just buy it from wherever and pirate it.

Purchase Fujitsu scanner, cut book up and place in ADF, scan for OCR and PDF the results.