If you've ever had to share Microsoft Office documents, you'll know the frustration when people open them in programs other than Microsoft Office. In an attempt to slightly ease the pain, Apple has provided a check sheet of features that it supports on the iOS version of iWork.
As usual, the simpler your document is, the better; things such as tabbing in Word documents can play havoc, bidirectional text and ligatures aren't supported, and neither are headers or footers. In many cases, fancy versions of charts are knocked back to basic line and pie graphs. Excel docs lose their ability to freeze panes and lock cells, and don't support conditional formatting. Some of PowerPoint's fancier functions, like aspect ratio lock, comments, image adjustments and a lot of the ways that you can use links, are lost when importing into Keynote.
If you're preparing for multi-platform and have to use Office, check out Apple's site to make sure it'll look fine when opening in Pages, Numbers or Keynote.