Help: Acrobat 9--Problems using PDF/A file

I accidentally posted this on another part of the forum--I apologize for re-posting it, but I am getting desperate. I hope someone can help.
 
I have Acrobat 9 Pro and some of the people I collaborate with have Acrobat 7. We share PDFs of articles from medical journals and often need to annotate them (underlining text, drawing boxes, etc). Sometimes I will receive a PDF from a coworker that appears as a PDF/A when I open it. In such cases, most of the Commenting tools are grayed out so I can't underline, box, or other wise annotate anything. However, it turns out that my coworkers with Acrobat 7 don't have this problem--when they open the file it's not a PDF/A for them and they can mark it up just fine.

Is there any way I can turn off the PDF/A mode? I've browsed through the menus and toolbars trying anything that might work and I am officially stumped. I'm pretty much a novice with Acrobat--I only use a few features--so please go easy on me when you try to explain.
 
Or maybe there is some way my coworkers with a lower version of Acrobat can save the PDF in order to avoid this problem?

Thank you so much for any help you can give.

Signed,

Writer on a Tight Deadline
(Dianne)