"eTextbooks" on the Kindle and Calibre.

[Gaaks]

Hello everyone, does anyone in there understands how these "eTextbooks" work? I got one here and they're almost like "pdf files but better" on the kindle, you have some options like "fit to screen" and zooming is faster and more manageable, plus with fit to screen you can basically read everything in this textbook I have.

Using calibre I figured out that the book is an azw3 file, which explains the better features a bit, and after removing drm you can convert it to a pdf file, and it then becomes pretty much a normal pdf if you crop the borders it ends up with after conversion, and when I went to edit the file, turns out all the pages are simply jpeg images.

My main curiosity is, can I convert a pdf into such a file? Maybe it wouldn't work out with Calibre specifically, but seeing as this is a conversion sub forum, maybe some of you would have ideas on how to do such a thing?