Ensure App is free & included as apart of joining estimate one

Keep it simple:
Allow for Notification system -
subcontractors notified when new tender requests, correspondence & notification of changes.
Allow for simple response for tender requests - Quoting or Decline.
When opening correspondence mark as read for head contractor to see as apart of audit trail.
Provide Reminders - 24 hours prior to quote is due if selected quoting
Don't go to extent of being able to download or view docs in app -makes app complicating and could cause app to be unstable due to size of documents etc rather provide weblink like provided currently in email that opens estimate one in an internet browser.

Provide Linking system with address book -
Send & accept Request system as well as easy block etc.
Allow for self manage easy update contact details which updates details to all linked contacts.
Include subcontractor to be able to self manage there trade(s) they want to quote for.