To Link Existing Document to Project:
1.Select a document in the Projects > Documents tab.
2.Click the Show and Manage Links to Selected Document icon.
3.Click the Add New Link icon, 
.
4.Select one of these options:
•Add Links to People
•Add Links to Companies
•Add Link to Project
•Add Link to Job Order
5.Find and select the object.
6.Add the required Link(s).
7.Click OK.
8.Click Close.