How to Underline Text in a PDF on Mac
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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.
Similarly, you can also highlight and strikethrough text in PDFs.