If you’re using an Android smartphone or tablet that has a display measuring 10.1 inches or less, you now qualify to download and use the stable Mobile Office 2013 versions of PowerPoint, Word and Excel for free, forever. You can find these apps inside the Google Play store and if you wish, they can be integrated into your Office 365 subscription to form a more cohesive experience with your desktop or laptop computer. If you want to use these apps on larger devices like tablets, you’ll have to pay for the privilege of using advanced editing functions only available on paid versions of Office 2013. In addition to these three, Microsoft also offers OneNote and Outlook on Android devices and currently has all of these applications for mobile use available on the Apple App Store for iOS devices as well.
Source: Office Blog