RTF: Actually, I personally like RTF for much of the same reason I like HTML: It can be read on a variety of word processing programs and most OSs (but not all) come equipped with a compatible word processor: Word, WordPad, AbiWord, OpenOffice, etc.
The problem with RTF is that image files embedded in RTFs are HUGE.
But RTF is great for text only books.
I also like txt (you can do formatting notes, just clarify them in introductory text, like brackets mean /italic/ and asterikseses
mean *bold* ... but again, not so good for anything with illustrative matter.
I've gotten used to PDF...but the fact that PDF is generally built with a specific screen size in mind means that you often get wonky results if reading on a non-standard screen and I often have to resize to suit my screen to the point where reading a PDF is a constant combination of scrolling and clicking, scrolling and clicking...plus load times for new pages tend to be fairly slow on my PCs PDF reader. PDF is tolerable, but it really is designed to be printed out for the most part, at least in my experience.