Find your file in the Dropbox app, then tap the "..." in the top right corner of the Dropbox app.  Then click "Share", then "Export...

You can access your input and output files by tapping the "History" option on the first screen when you open the app. From there you can share or ...

In the Mail app, Touch and hold the attachment until a menu appears. Tap the share icon (up arrow inside a rectangle), then choose the app you want to ...

Simply go into the app where your file is, and look for an "Open" or "Share" button. The buttons usually look something like this: Next...

If your file is on your PC, you will first need to get the file onto an app on your phone (such as your email or Files app), then follow the steps for "...