Cloud-based services like Google Docs have replaced old-fashioned word processors for some of us, but plenty of people still want to open Microsoft Word files. So many, in fact, that Microsoft Word for Android has hit the 500 million install milestone in the Play Store. That's the first app published by Microsoft (not counting Skype) to reach that number.
For a long time, Microsoft kept its full Office apps off competing platforms like Android. When it became clear that Windows Phone wasn't working, it branched out to Android and iOS. Word is ahead of the other Office apps like Excel and PowerPoint, which are still back in the 100 million or more category. Considering how many Word files show up in my email, that's not a surprise.
The Word app is free to download, but some of the features won't work unless you have an Office 365 subscription. Paying up adds features like Track Changes, page breaks, headers/footers, and more. It's still a perfectly serviceable way to view and edit Word docs on your phone with just the free account.