from the tech point of view you have an open platform to share and annotate scanned docs. Sort of "web 2.0 Sharepoint".
The niche that I see for such a service is legal. Lawyers can receive, annotate and remark on clients drafts for proposals, contracts etc.
So it's something like Scribd+ or Docstoc+, for closed network of service provider & his/her clients, where a lot of paperwork is required. Legal services seems like the case.