Hi Susan,
For us, once a project is Complete, that is archived for us. We keep everything in Workfront so we can do trend reporting. There are a few offices that require their PM to download all the documents to put on our internal client work server.
There is no storage limit on Workfront objects and because we have so many users, we don't really have to worry about document storage (but you might want to follow up with me on that in a few years. ;) )