Erro conhecido.
Microsoft Internet Explorer 9 was released after development for Adobe Reader X was complete. While we made every effort to anticipate the way the final release version of IE9 would behave, our PDF viewer plugin is not yet compatible with IE9. This is being worked on by our engineering department. In the interim, when trying to open a PDF file from the browser, save it locally to your computer and open it directly in Adobe Reader.
Moving forward, please continue to install updates to Adobe Reader X as they become available. When this issue has been resolved, it will be pushed via an update. Thank you,
David Acrobat Community Manager Adobe Systems
Vi que isso pode ser resolvido garantindo que os usuários executem a versão de 32 bits do IE, mas YMMV.