This service is in beta but already promising.
You can upload and share any file type ( size and extension ) from your computer or cloud services after signing up, and let your collaborators or pupils react, comment and annotate directly on the document.
What’s more, “versioning” capabilities help keeping track of the various stages of your project.
Keep in mind that your documents will be live for 60 days before deletion…
On top of it all, iOs and Android apps are available ! Awesome !