We've all been there before: You're at the airport, writing up a report on your laptop when you realize that your battery is running low and you left your charger at home. If you save all your files to your Dropbox, though, all's not lost. With the Dropbox mobile app, you can pick up right where you left off - and keep chipping away at your assignment. Here are three ways to stay productive with Dropbox on your phone or tablet.