The document message bar appears only in certain types of PDFs. Typically, you see this area when you open a PDF form, a PDF that has been sent to you for review, or a PDF with special rights or security restrictions. The document message bar appears immediately below the toolbar area, and can be hidden or shown by clicking its button on the left side of the work area.
Look on the document message bar for instructions on how to proceed and for any special buttons associated with the task. The bar is color coded: purple for forms, yellow for reviews, and blue for certified or secure PDFs.