If you have an active Office 365 subscription that includes the Office desktop applications, you can download Office Mobile for Android phones from the Google Play Store and use it to access, view, and edit Office documents (Word, Excel, and PowerPoint) on your mobile device.

License rights to Office Mobile for Android phones are included with the following Office 365 subscriptions in Volume Licensing: Office 365 Midsize Business, Office 365 Enterprise Plans E3 and E4 and Office 365 ProPlus (including Education and Government Office 365 subscriptions with Office desktop applications).

In order for Office Mobile for Android phones to work on your device you must have activated one of the above Office 365 subscriptions in your organization. To associate Office Mobile for Android phones with your Office 365 subscription, just sign in with the same user name and password that you use for your Office 365 subscription. Note that if you cancel your subscription, the app will stop working.

Your Office 365 subscription purchased through Volume Licensing gives you the right to access and use Office Mobile Apps on up to 5 smartphone devices.  Microsoft may add additional terms and conditions to these license rights at any time by updating the Product Use Rights and Online Services Use Rights.  Such updates will govern your access and use on and after the date of publication. 

Windows Phone devices includes a pre-installed version of Office Mobile that you can use, to access, view, and edit Office documents. An Office 365 subscription is not required.

For additional information read more at http://blogs.office.com/b/office-news/archive/2013/07/31/office-365-subscribers-get-office-mobile-for-android-phones.aspx