Can you higlight a region (*not* text) in Preview?
I could not find a way to do this using Preview.  I tried to cheat by using the line drawing tool, and setting the color to a transparent yellow.  Unfortunately, Preview doesn't seem to allow transparency with its shape tools. (To see what I mean, draw a line, then bring up the color picker window and move the Opacity slider around -- it's either all or nothing.)

As an alternative, I suggest Adobe Reader (I receive no commission).  Its text highlighter has the same limitation as Preview's, but its line drawing tool supports transparency. Open up the Comment pane on the right, then the Drawing Markups section. Right click on the line tool and select Tool Default Properties to change the lines' color, opacity, and thickness. You also might want to enable Keep Tool Selected so you can quickly highlight multiple areas. 




Instead of the line tool, you might prefer the rectangle tool.  Follow the same steps as before, but additionally set the Fill Color to be the same as the Color (and make sure the Transparent checkbox is NOT selected for the Fill Color).