How do I add a document to a completed task?

Go to the module History.
Select Maintenance.
Click on Search to search for the relevant maintenance task.
Once you have found the maintenance task, there are two ways to add the document:
Right-click on this maintenance task and select Add document.
Double-click on the line or click View and then New to add the document.
The option Link from documents... makes it possible to add a document from the module Documents.