True to its current motto of being the 'productivity company,' Microsoft has continuously improved the experience of Office 365 users using the apps on non-PC or Microsoft devices. The latest update to Microsoft Office for iOS allows users of Word, Excel, and PowerPoint to save their files on iCloud (as well as the de facto OneDrive).
This works by allowing third-party cloud storage providers to natively integrate storage services into the Locations picker in the Office apps (Word, Excel and PowerPoint) for both iPad and iPhone. iPhone and iPad users will now see iCloud as well as Box as options. This openess to cloud storage services is likely coming to other Office 365 products on Windows 10 and Android.
Source: Thurrott