Text Highlights

Use the text highlight button to add a coloured, semi-transparent highlight around selected text. Like other annotations, the highlight has an associated pop-up note window.

Select the Text tool () from the tool bar.
Select the text you wish to highlight.
Press in the formatting tool-bar to highlight the selected text
or press Ctrl+G.
To open the associated pop-up note window, double-click the text either the Text tool () or Hand tool ()

Notes

Text highlights are annotations that sit on-top of the PDF page like other annotations. They do not track the text to which they apply which means that if you edit the text on the page after highlighting, the text may move with respect to the highlight.
A similar result can be achieved using the underline facility - this will track edited text but does not have an associated pop-up note window. See Underline & Strike-through