1. Open the PDF document in Highlights
From the menu bar choose File > Open. Locate and select the file you want to open, then click Open:
The PDF opens in a document window:
2. Select the text you want to underline
This will bring up the selection popover:
3. Choose a color and click the underline button in the popover
Your annotation is added to the PDF and the text you underlined appear in your notes which you can easily share.