The highlight tool can only highlight "real" text. If you have a scanned document, then you cannot use this tool. You can tell if it's a scanned file by zooming in really close on some text. If the text gets blurry or loses sharpness, then it's likely an image. Real text stays sharp and crisp at any zoom level.
If you have any other questions, please send us your document with an explanation of the problem via [email protected] if possible. We will provide you with special customer support.