There are various ways of accessing and editing documents, spreadsheets and powerpoint presentations on the iPhone, but Apple's own productivity apps, Pages, Numbers and Keynote, are now available on the iPhone (and iPod touch).
Each app has had its UI optimised for the iPhone 4's retina display, which also includes a redesigned document management thumbnail system. Other features, such as AirPrint and multitouch gesture support are also present in the iPhone version of each app. Pages, Numbers and Keynote are available now in the app store as universal builds for $13 apiece. Those who bought any of the apps for the iPad can receive the iPhone version as a free upgrade. [Pages, Numbers, Keynote via Apple]