While technically, the iPhone & iPod Touch both support PDF files, it's a sub-par solution for reading on these devices (you can't store the files, constant need to pan & zoom etc.).
Several applications on the iPhone support better (reflowable) e-book formats and even include a direct access to Feedbooks.
Stanza is the most downloaded dedicated application on the AppStore and provides a very convenient access to Feedbooks. Stanza is a free application and is based on our EPUB output.
Just open Stanza, go to Online Catalog and select Feedbooks.
Download this application on the iTunes App Store.
Bookshelf also supports a direct access to Feedbooks. It can read either EPUB or Mobipocket files downloaded from our service.