As a student, I can say that the point isn't always to read the whole book: the kind of textbook you'd want in a PDF form tends to be large reference books which you can search quickly in a PDF reader. But I'm used to working and reading whole books on the screen, and believe it or not there are quite a few legal ones: for example I work with a few art books whose copyright has expired a long time ago, and I read a lot of research papers in PDF.