With the Reference Manager, you can view PDF underlays that are attached to a DWG file and edit the paths of the attachments.
With the Reference Manager you can check any file dependencies that a drawing file may have. You can also load and unload underlays and perform various other operations. The properties reported by the Reference Manager include
You can view the list of references by drawing or by reference type.
If an underlay’s status is Not Found or Unloaded, it displays in the drawing as an underlay boundary box.