Use your own free Dropbox on each device:
- On your first device: Settings > Advanced > Storage, then tap Connect on the Dropbox card and sign in.
- On a second device that's brand new: during setup choose Import from Dropbox and connect the same Dropbox account (don't tap Start new, that makes a separate workspace). A device that's already set up can connect from Settings > Advanced > Storage instead.
Now your work syncs through Dropbox automatically whenever you're online. Make an invoice on your phone and it shows up on your computer, and the other way around.
(Live, real-time sync for a whole crew editing at once is a ToolBerry Pro feature.)