Students, and ofcourse other people, often use ebooks these days which are available in many different formats including PDF PDB CHM etc. Ubuntu doesn't support all out of the box. Plus one cannot use these books interactively; one should be capable of opening an ebook and then should be able to make notes, bookmark things, underline/highlight things and do all kind of activity required by a student. Since a lot of people use ebooks and with the advent of netbooks & tablet PCs, this use is going to increase.