Take for instance the books you can get here:
http://www.apress.com/info/dailydeal
I'd like to read them on my Cybook, but they are only available as PDF, plus password protected. You need to remove the protection (I hope this is allowed for personal use, but I'm not even sure) and convert them to a format that is readable on the Cybook. These technical books often contain tables, code sections set in different fonts, or images, which all must be properly converted. So it's not so simple.