But, when they try to do so, they found that the highlight tool is not working in PDF file. Most of the times while reading the journals, magazines or e-books users find some important points or sentences that need to be highlight. Highlighting the PDF text (words, phrases or paragraph) is very useful to mark up text for future reference or for the reviewing purpose. Can anyone tell me why I can’t highlight text in PDF file? But, I have found that the highlight tool is not working in PDF file. I was trying to select & highlight text in PDF document with Adobe Reader DC.