Technical note - PDF isn't the most popular and universal format for text exchange, nor does it necessarily display the same way on all computers (that's the
idea, yes, but in practice...). The "Small file size" comment is also very misleading.
I'm not gonna go on a big rant about how much PDF sucks for anything but physical printing, because I'm sure you've
heard it all before. However, I will say this: PDF should be offered
only as an alternative to your main format, for people who want to print the eBook and have a physical copy. Your main format should be either HTML (readable on every computer, every PDA and most mobile phones) or, better yet, plain text (readable on every computer, every PDA, most mobile phones and most MP3 players). Standard convention for italics in plain-text files is to use the underscore key, _like this_. If your readers can download your eBook and then read it on their crappy, twenty-quid-from-eBay Chinese MP3 player while waiting for the bus, then you're onto a winner.
In short, the best thing to do is to provide your eBook in three formats:
1. (X)HTML, with the CSS (if any) defined in the head, for reading on a computer. Have a feature that lets the user switch between black text on a white background, white on blue, and light grey on black, to optimise for onscreen reading.
2. Plain text, using _'s to denote italics, for reading on a low-tech device such as an MP3 player (I believe iPods have this feature too).
3. PDF, for the tiny minority of users who will actually want to print the eBook.