Commenting tools are available only in PDFs that have commenting enabled. PDFs in a review workflow typically include commenting rights.
You can use the Highlight Text tool, Cross-Out Text tool, and the Underline Text tool to add comments by themselves or in conjunction with notes. The Cross-Out Text tool and the Underline Text tool don’t appear in the Comment & Markup toolbar, by default.
You can add a highlight with a note or you can cross out text by selecting the text using the Select tool or Text Edits tool, and then choosing that option from the menu that appears. However, if you’re marking up a lot of text, the specialized tools are faster and easier to use.
More info on 7 here: http://www.adobe.com/products/acrobat/acrrfaq.html