What is this going to cost me?
To download the app? Why, it's free! Downloading Office Mobile won't cost you a dime.
Using Office Mobile, on the other hand, will require you to subscribe to Office 365. That's Microsoft's cloud-based subscription offering, and the Office 365 Home Premium edition costs you $10 a month or—if you pay up front for an annual subscription—$100 per year. (You can use Office 365 for free for 30 days, though.) Business users have their own Office 365 pricing.
Making an Office 365 subscription a condition for using the iPhone app certainly makes Microsoft's cloud-based offering more attractive to would-be subscribers. Microsoft has even said as much, with Pat Fox, a senior director of product marketing in the Microsoft Office Division, stressing in the Office for iPhone announcement that the new app is "adding even more value to our Office 365 subscriptions." And, in a world where the likes of Google and its Web-based Google Docs are making business users reconsider their need to have something like Office, Microsoft is properly motivated to want to goose Office 365 adoption rates.
Of course, tying Office Mobile to an Office 365 subscription may also have the unintended consequence of making Apple's mobile versions of iWork seem more attractive. Apple's word-processing, spreadsheet, and presentation apps for mobile may cost $10 each, but they work natively on both the iPhone and iPad, even if you don't have their Mac equivalents installed on your computer. (Having the Mac versions of Pages, Numbers, and Keynote will, however, help you get more out of their mobile counterparts.)
On a related note, if you wonder why Apple took the time out of its busy WWDC schedule to note that a cloud-based version of iWork is coming soon (along with updates to the Mac and iOS versions), wonder no more. Apple obviously knew that a mobile version of Microsoft Office was on its way to the App Store and wanted to preemptively remind people that it's got a few products of its own for on-the-go productivity mavens.
Why is this only available for the iPhone?
Well, Microsoft has these Windows-8-powered tablets that it's trying to sell. Sales aren't going particularly swimmingly, so it's not in Microsoft's interest to take the one advantage its Windows tablets have—dedicated versions of Office—and extend it to the iPad, otherwise known as the tablet people are actually clamoring to buy. After all, when your latest ad campaigns tout your Office advantage, you're not about to turn around and push out a version of Office for the iPad. (Of course, you could always run Office Mobile on your iPad if you wanted: Like any iPhone app, it will run in a compatibility mode with a less-than-optimal resolution. "You'll have a more satisfying experience using Office Web Apps," Microsoft said.)
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