We were just troubleshooting this on Monday. You are correct, it is not possible to set a unique identifier for a document in the enterprise (compared to a unique identifier within a project) through the general UI (I am pretty sure it is possible through reports, but that's not really helping in the general user sense). Sorry Karen, if we figure out a workaround we'll post up.
For those that are interested in unique ID's for single project items:
For some reason the Item Number custom field is not exposed for native applications (tasks not relevant). So as a trick, try creating a custom calculation field (in Documents for example):
Admin>Applications>Documents>New Custom Field
- Title it "Document Number" or whatever your ID number should be. Not required is fine.
- Set it as a Calculation
- Standard settings are fine, maybe set zero decimals
- In Step 4, highligh ID and "Add" it to the forumla box and save.
Whenever you add a document to a project it now exposes it's ID in the calculation field. The first doc you add is 1, second is 2 and so on. Updating a version of that document retains its original ID number. This can be displayed in the details screen or the documents tab as a field.