Change the PDF/A viewing mode

PDF/A is an ISO standard for PDFs. Documents you scan to PDF are PDF/A-compliant. You can specify whether you want to view documents in this viewing mode.
In the Preferences dialog box under Categories, select Documents.
Choose an option for View Documents In PDF/A Mode: Never, or Only For PDF/A Documents.
You can switch in or out of PDF/A viewing mode by changing this preference setting again.



http://help.adobe.com/en_US/Acrobat/9.0/Standard/WS3E0E8467-B787-4020-A1D3-6BC762A42DF2.w.html