I was recently given an ebook by a friend. It was a photography book, with tons of hi-res images and very little text. When I opened it with Ubuntu, Evince (the default PDF viewer that comes with Ubuntu) gave in: after a few pages, it slowed to a crawl. I did a bit of research, and found the program that rescued my viewing needs: MuPDF. The good news was that I could finally read my book. The bad news was that I found out that the company behind it has in the past misunderstood the terms of the GPL and started a (later withdrawn) litigation against Palm.