OpenOffice Document Reader lets you view all sorts of Office documents without paying one red cent. It won't work for everyone and is loaded down with ads, but it's still near the top as far as Office readers go.
OpenOffice Document Reader is just a reader app, so you won't be able to edit any of the files it works with. OpenOffice Document Reader displays text and HTML just fine. If it doesn't work with your file, it briefly displays it on a cloud server so you can read it. The reader slides animated ads into all of the files you try to read with it. Instead of searching your folders for documents for you to look at, it makes you manually find them. It can grab documents from Dropbox and share them to social media, which is a welcome addition to reader apps.
If you don't need an app that actually lets you edit documents, this one is just fine. It's as good as reader apps get, really. Since it doesn't slam you with ads at every corner, OpenOffice Document Reader is much easier to stomach than many other reader apps found on the Google Play store.