We are aware that syncing between different machines is a much needed feature we cannot yet officially support. Although we are working on improving this aspect of Papers2, there are a few workarounds for using the same Papers library across different Macs.
A word of caution before you get started: Both of the methods below require your Papers library to be located on a location other than your own computer hard drive. The problem that can arise from this is potentially lost or corrupt data, and we cannot stress enough the importance of making frequent backups of your files. Particularly when using one of these methods to access your Papers library across computers, you are at a higher risk of corrupted files or lost data. You can access more details about how to backup your Papers library in this article.
Saving your Papers Library to an external hard drive, and pointing Papers on your different computers to the same location on the drive, means you can use the same library across computers. Backup your library folder frequently on another drive, and use incremental backups to make sure you have a recent version of your library available if you are ever affected by file corruption or data loss.
To save your Papers library folder to an external drive, you can typically locate it at: /Users/your_name/Documents/Papers2 or /Users/your_name/Library/Application Support/Papers2
In Papers, just point Papers to your library folder on your external drive in the preferences. If you do this for each computer on which you want to use the same library, you can do so by carrying your external drive with you. The downside of this is that you do always need to have your external drive to access your library.
Syncing your Papers library with dropbox is not officially supported by Mekentosj. However, some users have described being successful in syncing via Dropbox and for this reason we offer the information here. Bear in mind that Dropbox is a third party service and you need to make sure you backup your Papers library in another location as well, and not solely rely on Dropbox for storing important data.
The concept is the same as described above: move your Papers library to your dropbox folder and point each computer to it.
When syncing with Dropbox there are two crucial things to keep in mind: NEVER run Papers on two macs at the same time that have their library shared via the same DropBox account. So you should ALWAYS quit Papers and Citations before moving on to another machine (the latter is the status item in the menu bar).
One last important thing to keep in mind: Before turning off your computer, make sure Dropbox is finished syncing so all changes Papers made to the database and library have uploaded to Dropbox successfully. If you turn off the computer in the middle of the sync, and you turn on your other computer and Papers try to access the database from there, this might overwrite the library to an earlier version or it can cause your library to become corrupted (which can only be fixed by replacing it with a backed up version).