As with the previous post below about Office Mobile for Android being free, it's been released for iOS devices free of charge as well. Same conditions apply as for the Android version; an Office 365 account is required for non-personal use.
- Sign in with a free Microsoft account to create, edit and save documents for home use. A qualifying Office 365 subscription is required to create, edit and save documents for business use. Qualifying plans include:
•Office 365 Small Business Premium
•Office 365 Midsize Business
•Office 365 Enterprise E3 and E4 (Enterprise and Government)
•Office 365 Education A3 and A4
•Office 365 ProPlus
Requires iOS 7.x
Edit: An Office 365 subscription is required to be able to edit documents on an iPad, but not on an iPhone or iPod.