Verify that you have set up the Document Management Folder in your global settings: Setup > My Company > Information & Settings > Documents.
This folder and the drive it is located on should be accessible to all computers and identified as the exact same path, i.e. “C:\PM Client Documents”. If you can browse to this directory, verify that the documents are located in the client’s folder. If they are not there, identify the staff member which added the file and ensure that they have the correct document path on their computer and they have access to it.