There are two ways locate the Document ID.
If the document is open, check out the Document tab of the Doc Info Tray:
In the General Information pane, the system-generated Document # is listed.
If the document is not open, check out the Search tab of the Search Utility:
1.Search for the document.
2.Highlight the document in the Results pane.
3.Click the 
icon located at the bottom of the screen.
The Document Information window similar to the following displays:
In the General Information pane, the system-generated Document # is listed.
