Microsoft released an iOS app named Microsoft Office Mobile 1.0 last year, which supports viewing/editing Word, Excel, and PowerPoint documents. Although free, it required an active Office 365 subscription to use.
Microsoft Office Mobile was recently updated to v1.1, and it no longer requires a Office 365 subscription for “home use”. Microsoft’s app description stays it requires “a free Microsoft account, is free for home use, and requires a Office 365 subscription for business use”. Basically it’s on the honor system now.
The updated Microsoft Office Mobile v1.1 is optimized to fit the iPhone 5 screen, adds many new features, and requires iOS 7. It should be noted that although this is an iOS app, and will run on iPads, it is not optimized for the iPad like the new iPad versions of Word, Excel, and PowerPoint.