To use an oval to highlight 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 Oval or Rectangle from the Annotate pop-up menu in the toolbar.

Drag over the area you want to enclose.

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 change the shape’s color, choose Tools > Show Colors.