Portable Document Format (PDF) is an increasingly popular standard for e-books (and hundreds of other uses) because of:
1) how well it preserves formatting across many platforms
2) and the security options it provides
There are other options, even free ones. But before committing to a cheaper product, find out what formats they convert from (one popular convertor ONLY works with text files) and whether they can make URLs clickable (many do not.) For a nearly complete listing of PDF conversion utilities, visit Planet PDF Tools List.