Open the PDF in a PDF reader or editor that supports highlighting
Select the text you want to highlight
Click the Highlight tool
Choose a highlight color if available
Save the PDF
On mobile, open the PDF in a compatible app
Tap and hold the text, then drag to select it
Tap Highlight from the menu
Save the changes
If the PDF is scanned, use OCR or a PDF editor with annotation support
If highlighting is not available, use a different PDF app or editor