To use a line to emphasize an area:

If the Annotate pop-up menu isn’t in the toolbar, choose View > Customize Toolbar and drag the Annotate pop-up menu to the toolbar.

Choose Line from the Annotate pop-up menu in the toolbar.

Drag where you want the line to appear.

Change its appearance by doing one of the following:
To make the line thicker or thinner, press the plus sign (+) or minus sign (-).
To add or remove a drop shadow, press S.
To add or remove an arrow from the beginning of a line, press 1.
To add or remove an arrow from the end of a line, press 2.
To change the line’s color, choose Tools > Show Colors.