May I know on which OS you are working with?
To underline text in PDF files on a mobile device (iOS or Android),
Open the PDF in Adobe Acrobat app
From the bottom tray tap Comments icon to bring commenting tool.
From here tap text underline tool (4th from right on Android) to select it.
Then drag over the text which you would like to underline.
To change the underline color, tap & hold the text where you have highlighted.
This will bring a popover for Color, Opacity & Clear. Change color
Let me know if this helps.