Add Rectangle and Highlight

First, click the page you want to add rectangle in.

The selected page will have a blue bounding box surrounding it.

Click to show the edit menu, then choose 'Rectangle'.

A transform box with rectangle will appear that can drag, rotate and resize.

Double-click the rectangle to edit Color and Opacity.

You may drag a half-transparent to overlap existing text for highlighting.

To remove the rectangle, press on the rectangle.