When the witness is using a tablet for viewing documents, you have the ability to have them mark up a document. First ask them to click the annotate icon Annotate Icon in the control header of the tablet app.

Then have them click yes to annotate. 

Then the witness screen should indicate that it is waiting for driver to confirm the annotation. The witness can't annotate until you say so. 

On your screen, you will see the request. 

If you agree, a copy of the document is made and added to the document list.

This is an annotated version of the original document. This allows each document to be marked with separate exhibit numbers.

Once the driver confirms the annotation, the witness can click the annotate button to add annotations.

Once the witness clicks to add an annotation, they can then select a specific color or line thickness in the toolbar. 

Using their finger to draw, the witness can make annotations on the current page. When they are done annotating, the witness can click "Save" in the header, and that annotation is immediately visible by all participants.

To delete an annotation, the witness can tap on the screen in the area of the saved annotation. This pops up a confirmation alert asking if they are sure they want to delete the annotation.