The new version Printer Pro application takes advantage of action extension. With the application you can quick print files via Printer Pro extension.
To print a PDF file or an image from the Dropbox application, follow these steps:
If you use any other iOS version before iOS 8 on iPhone or Printer Pro extensions do not work properly, to print a file from the Dropbox app, tap on the Actions button in the upper toolbar. Tap Open In... afterwards and select Open in Printer Pro.
The Dropbox app will be automatically closed and Printer Pro will be launched.