Offering ebooks in multiple formats

[cziffra]

I'm interested in offering downloads of ebooks on several of my sites. In order to make the content as accessible as possible, I want to offer a few different formats. Given the list of requirements below, what formats should I look into?

Any format I choose should:

-work on a variety of operating systems
-be easy to create and use
-have the ability to include images
-keep file size to a minimum
-be free to use, i.e., no licensing costs
-be a self-contained file or in a format where a large audience would already have software to open it (Adobe Acrobat would be an example of this.)

I've already decided to use pdf, as it fulfils my requirements. Any other suggestions are greatly appreciated.

Cziffra

P.S. This seemed like the best forum for this topic since it deals with content. I apologize in advance if this should have been posted elsewhere.