The iPad Pro is due for release sometime in November, but owners who want to use the iOS Office apps on the 12.9-inch will need to pay Microsoft for an Office 365 subscription if they want to edit any office files on the tablet.
While iPhone and current iPad owners can edit Office files with the apps for free, Microsoft confirmed to Ars Technica that the larger screen for the iPad Pro means that the tablet exceeds the 10.1-inch limit for a "true mobile device". Thus, the Office 365 subscription is needed for editing files on Apple's next tablet.
At the moment, the End User License Agreement (EULA) for the Office iOS apps does not mention this screen size restriction, but it's likely that Microsoft will update it before the iPad Pro is launched.
Source: Ars Technica