It’s for only those who will subscribe
It has been long over due, and finally Microsoft’s Office plans for iOS, have been revealed and has delivered a copy for iPhone today. Unfortunately Office Mobile for iOS will be strictly iPhone only initially, and the other catch is that access to the application is only through an Office 365 subscription. There’s no free standalone version, nor an iPad edition.
Basically the app is designed for on the go editing. There are currently no plans for Microsoft to create an iPad version. As usual Office for iPhone is available immediately for US users, and it will roll out internationally in the coming days.
source: The Verge