Just set the preference not to use PDF/A mode File->Preferences
PLEASE DON'T DO THIS!! That preference is set the way it is in order to ensure that PDF/A files are processed by Adobe Acrobat/Reader according to the rules of the PDF/A standard. If you change that preference, there is a chance that the PDF will display quite incorrectly. See <http://blogs.adobe.com/pdfitmatters/2010/03/details_matter_conforming_with.html> for details.
The correct answer to the question is to use Preflight's "Remove PDF/A Information", which will turn the PDF/A document back to a standard PDF.
I reciently had this thing pop up on my software. I found on line to change the preferences to view in PDF/A mode to Never. I had to close all the files first and then open Adobe Acobat X, then go to my preferences. I cannot tell you how unhappy I am with this program. But I am stuck with it. It costs too much and does too little of what I need and too much of what I don't need... On top of that it is bulky as .... You know.