Do you have a Papers 3 library on your Mac that you would like to share between platforms and devices?
These instructions outline how to set up syncing of your current Mac library.
Open Papers 3 and go to Papers>Set up Sync.
Select Set up.
A popup will appear. Please log in to Dropbox and authorise access.
Papers will now move your library into your local Dropbox folder.
In the future, you can manually sync using the Sync Now button under Papers>Preferences>Sync (or Papers>Sync now), as well as set your preference to sync automatically.