The integration makes it possible to edit Office files from the Dropbox and access them directly from the Office apps.
To get started, make sure your Dropbox app is up to date on your iPhone, iPad, or Android phone, then open any Office doc, spreadsheet, or presentation in your Dropbox. Tap the new Edit icon (shown above) to start editing in the latest Office apps. When you’re done, your changes will be saved back to Dropbox automatically.
You can learn about how to use these new features here.
You can download Dropbox from the App Store for free.