We tested succesfully iFrame under Mac (safari) and Windows (IE and Firefox) http://code.google.com/p/flex-iframe/
Although the component is meant to show mainly html pages, you can also embed pdfs but not to render pdf on the client side. We only found a limitation for using "load indicator" when pdf files are embeded as Acrobat pdf reader does not triger the "load" to the html page container (issue number 38 for iframe) http://code.google.com/p/flex-iframe/issues/detail?id=38 HP
