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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. But it is not clear which app you actually use.)
It sounds like you have a different problem. 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.
Your problem may be caused by the type of your PDF document.
Here are the most common cases.
(a) Scanned PDF document containing no real text
Try selecting text instead of highlighting text. If you cannot select text at all (no matter how hard you try), your PDF document is likely a scanned PDF. You can only add Text Markup annotations (Highlight, Underline, Strikeout) to real text, not to image of text. You need to run OCR (Optical Character Recognition) software to turn images of text into real text.
(b) Secured/protected PDF document
The security settings or restrictions of the PDF document explicitly disallow commenting. The author/distributor of the PDF document does not want you to add comments to it. All of the commenting tools will be disabled in this case.
(c) Encrypted PDF document
When security settings (e.g. password, certificate) or restrictions are added to a PDF document, the document becomes encrypted. Unlike the desktop version of Adobe Reader, the mobile Adobe Acrobat Reader apps (for iOS, Android, Windows Phone) have the known limitation where changes cannot be saved to encrypted PDF documents. Therefore, users are not able to make any changes to encrypted PDF documents. The easiest way to work around the problem is to use the desktop version of Adobe Reader.
Pleas let us know if you would like us to examine your PDF document to identify the particular reason why you cannot highlight text.