For most documents the "Print" button can be accessed from the action menu when viewing a document.
Printing requires iOS 4.2 or later and a multi-tasking capable iOS device.
Important Note: printing is only supported for iOS compatible "AirPrint" printers. If you don't have a compatible printer on your local network then you won't see any available printers. Please see Apple's technical support http://www.apple.com/support if you're having problems detecting printers on your network.