Open Edit PDF text and choose your PDF. The editor opens with big coloured buttons at the top: Edit text, Add text, Picture, Sign, Highlight, Erase, Draw, Shapes, More.
Tap Highlight, then drag over the words you want to mark in yellow.
Made a mistake? Tap ↶ at the top to undo (↷ to redo). Nothing changes your original file until you download.
When finished, tap ⬇ Download at the top to save the edited PDF.