It is also worth noting that Dropbox saves its own revision history automatically, just like Google Docs.
Packrat is a Dropbox Pro add-on feature that saves your file history indefinitely.
Dropbox saves a history of all deleted and earlier versions of files for 30 days for all Dropbox accounts by default. If you have the Packrat add-on, Dropbox saves those files for as long as you have the Packrat add-on. With Packrat, you never have to worry about losing an old version of a file.