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Have you tried to activate Reader rights in Acrobat?
Is this happening for all you PDFs? When you open a PDF, do you see a lock icon in the Navigation Pane on the left hand side? If so, your PDFs have been secured to disallow commenting.
For Reader X, sticky notes and highlighter type of annotations are by default available and can be added on any pdf unless that pdf is secured (or has some special permissions due to which commenting has been disabled.). If you wish to enable all the types of annotations then you need to do the following. Open the pdf in Acrobat (any version). Click Save As->Reader Extended Pdf->Enable Commenting and Measuring.
Hope that helps.