Very often documents are connected to actions, e.g. "contact these guys", "pay this bill", or "discuss this with the team". You can create a 43 folder setup in DEVONthink (see the "Getting Things Done" concept) to remind yourself of them or manually add tasks to your preferred calendar or task manager, e.g. iCal, OmniFocus, or Things.
But why manually? DEVONthink Pro (Office) comes with scripts for Calendar and all major task managers that add references to your task list or GTD inbox including reminders. You can find the scripts in Scripts > Reminders.