If you head over to any of the Play Store listings for Microsoft's previously tablet-only Word, Excel, and PowerPoint apps for Android, you'll find they have a new feature for beta testers: phone support.
Microsoft has renamed the apps appropriately, dropping the "for tablet" moniker that they previously had, and the UI has been optimized for the smartphone form factor. To get the betas, join the Microsoft Office Preview community on Google+, then go here for Word, here for Excel, and here for PowerPoint.
If you're not seeing compatibility with any or all of the apps for your phone, just give the Play Store 10-15 minutes to populate. Eventually, as long as your phone is actually compatible (Android 4.4+ seems to be the only requirement), all three apps should install just fine.
We're still getting acquainted with the new features, so we'll update this post if Microsoft releases any more information or details.
Microsoft has published a blog post discussing the release in a bit more depth.