Be very careful using Dropbox to store Scrivener "projects" as in the example of collaborating across platforms.  Dropbox handles a project not as a file, but as a folder.  It is possible to lose a lot of work if you save a project to Dropbox in hopes of opening it on a computer other than the one from which it was saved.  

Instead, back up the project to Dropbox, selecting the "zip" option.  Then Dropbox will have but a single "file" to deal with and the integrity of your project will be ensured.  To open it on a different computer, first download the zipped backup from Dropbox, then extract the project and work on it, then back it up to Dropbox as a "zip" file once again.

Don't make the mistake I did, not comprehending that a Scrivener project is not a file, but a folder with complex indices.  I lost many hours of writing as the fruit of my ignorance.  It cannot be recovered.