Would be good to have the ability to check documents out when you are working on them so that all other users only have read only access to the document.
In addition to the check-in and out the ability to layer documents would be ideal. Each file can have various layers for the same document. Checking in as a minor version would be 1.1(Original upload would be 1.0) and checking in as a major version would promote the document to 2.0. This function is important to maintaining history of PDF documents. Please feel free to contact me for further explanation.