The professor is giving a class, they're going to teach all of what is in the textbook, and for those who can't write it themselves and use ones written by other professors, they can give out that PDF, they don't have to write one.
I was simply responding to this:
> It's not like there is a cheap alternative that the professors can switch to. All new edition textbooks are expensive. They can't specify out of print editions because then you can't guarantee that your students can get access to them, and the textbook companies update the problems in the books just enough to make using a different edition from the one specified impractical.
in the parent comment. I'm pointing out that there is a cheap alternative. We have had digital documents for decades, there are ways to keep those documents up to date without rewriting the entire document, there is no excuse to have a publisher and distributor for educational companion text documents for students in classrooms. It is glaringly obvious that the status quo with textbooks is artificially maintained even though technologically there is absolutely no reason it has to be this way.