How to Underline in a PDFby Kenneth Falker
Underlining a portion of a document gives that section additional attention. You can also use underlining for certain sections, such as chapter titles or significant quotes. A .pdf document may come with previous passages or sections underlined, but you can also use Acrobat to underline your own selections for specific areas that demand more attention.
Click "Tools" on the Acrobat menu bar, point to "Comment & Markup" and then click "Underline Text Tool."
Press "Ctrl" + "E" on your keyboard at the same time to display the "Properties" toolbar, which allows you to customize certain aspects of your underline, including color, thickness and line style.
Highlight the text you want to underline. To stop underlining, turn the tool off the same way you enabled it.