The security settings or restrictions of the PDF document explicitly disallow commenting.

You need to run OCR (Optical Character Recognition) software to turn images of text into real text. You can only add Text Markup annotations (Highlight, Underline, Strikeout) to real text, not to image of text. If you cannot select text at all (no matter how hard you try), your PDF document is likely a scanned PDF. Try selecting text instead of highlighting text. (a) Scanned PDF document containing no real text Your problem may be caused by the type of your PDF document. The other users in this forum thread had questions about how to find the commenting tools in the new mobile Reader app - Adobe Acrobat Reader for iOS or Android. It sounds like you have a different problem. But it is not clear which app you actually use.) Are you using the desktop app on a Windows/Mac computer or the mobile app on an iPad/iPhone or Android tablet/phone? (Your forum post appeared in the mobile forum.