Using the commenting and text annotation tools depends on your PDF being text-enabled. Not all PDFs are, and if the document was created through a scanner, it is certainly not text-enabled. Fortunately, Acrobat is able to convert 'flat' documents in to editable ones through a process called OCR - Optical Character Recognition. Follow the steps below to learn how to use OCR on a document.
1. To run OCR on a PDF, open in Acrobat X and expand the Tools pane. Choose Recognize Text and then either 'In This File' to convert one, or 'In Multiple Files' to convert documents in bulk.
2. In the window that opens, confirm that the settings are the way you like (the defaults are English, Searchable Image, 600dpi), or change them by opening the Edit menu.
3. Click OK and OCR will run. You will not have to save or reopen your document for this to take effect; once the wizard process your document, you will be able to perform whatever text-enabled functions you wish.