I'm using a digital book for my class. Why doesn't the library just give students the PDF?
The Library is providing controlled digital lending (CDL) of certain textbooks and other library materials. In order to comply with copyright laws, the Library cannot make digital book PDFs freely available for printing and permanent download. For this reason, there are limitations in place regarding how digital books can be viewed (through a viewer) and how many people may view a digital book at a time.
If you're curious, here's a site where you can learn more about controlled digital lending by libraries.