After many clicks, I reach a link to a possible solution to this question:
How do I have marking tools appear on a student's submission? On one student's work they may appear. On the next, nada. Once the 'answer' was apparently within reach the reply was that I do not have sufficient access.
It depends on the submission type you are allowing for the assignment.
Text entry does not have annotation tools with DocViewer, but most file submissions do.