Aside from e-journal downloads referenced in the article, I'd guesstimate that at least 50% of the coursebooks I used over the last year were electronic copies from services like IBUK (Polish online textbook repository), Springer or Wiley (and some other that escape me at the moment). It's often more convenient overall, and I don't need to worry about late return fees or lug them around, hoping I won't lose or accidentally damage them. Most importantly, when I need one, it's always there, and I don't need to sign a waitlist that's sometimes three years long for a rare book with maybe one or two relevant chapters. If the book was published after 1990, there's a stupidly high chance that I can download/access it legally from a computer.
Also, you can do a word search in a PDF, something that physical books sorely lack. Programs like Okular even include tabs and bookmarks with comments/keywords, making the review process that much easier.