Microsoft Office was conspicuously absent from the iPhone's app list until now, but now that it's finally arrived, there's a catch.

Apple is a very popular brand and its iPhones and iGadgets draw a sizable fan base, but some apps such as Microsoft Office were still missing in action. On Friday, June 14, Microsoft released Office Mobile for iPhone in the iTunes Store, but there's a catch.

The app is available as a free download, but Office Mobile for iPhone will only work for users who subscribed to Microsoft's Office 365 Home Premium and ProPlus service. It's also worth noting again that this app is for the iPhone, which means that it's no optimized for iPads. Office 365 costs $99.99 for a one-year subscription.

"Microsoft Office Mobile is the official Office companion optimized for your iPhone. You can access, view and edit your Microsoft Word, Microsoft Excel and Microsoft PowerPoint documents from virtually anywhere," reads the app's description. "Documents look like the originals, thanks to support for charts, animations, SmartArt graphics and shapes. When you make quick edits or add comments to a document, the formatting and content remain intact."

Why does Office Mobile for iPhone require an Office 365 subscription? Because Microsoft did its best to leverage its popular Office to grab some more market share in the highly competitive smartphone market dominated by iOS and Android devices. Office comes pre-installed on smartphones running Microsoft's own Windows Phone platform, but iOS users can't have it that easily.

Windows Phone accounted for just 3.2 percent of the worldwide smartphone operating system (OS) market share in the first quarter, while iOS held 17.3 percent and Android gobbled a massive 75 percent.

Office is an important leverage and Microsoft knows that. Limiting the potential market for Office Mobile on iOS not only helps Microsoft keep its leverage, but it also helps boost Office 365.

Office Mobile for iOS is currently available only in the U.S., but it will reportedly roll out globally next week. The app offers access to Word, Excel and Powerpoint and the files will sync with Microsoft's SkyDrive cloud storage service. Users will not be able to store files using Office on Apple's own iCloud Web storage service.

Several features, meanwhile, allow Office 365 subscribers to pick up files on their mobile phones exactly where they left off on their PCs. Tapping the "Recent" button on the bottom of the Office app's home page, for instance, will display a list of all the Word, Excel and PowerPoint files they recently worked on from their PC, phone or tablet.

With the addition of the new app for iOS Microsoft Office is now available for Windows Phone smartphones and iPhones, granted that owners of the latter have an Office 365 subscription. Android is currently not invited to this party and if it will, the app will likely involve similar limitations.