To highlight a section of a PDF document:

Choose Tools > Annotate > Add Oval or Tools > Annotate > Add Rectangle.

Drag within the document until the shape is the size you want, and then move it to the position you want.
You can emphasize especially interesting or important parts of a document by enclosing them in an oval or rectangle.
Choose Tools > Annotate > Add Oval or Tools > Annotate > Add Rectangle.
Drag within the document until the shape is the size you want, and then move it to the position you want.