JQuey UI selectmenu goes behind iframe with PDF

JQuey UI selectmenu goes behind iframe with PDF

I have a page (see attached image) that contains a jquery ui selectmenu along with an iframe below it.  The iframe is loaded dynamically via a servlet and can contain either plain HTML or a PDF document.  The problem I'm having is when a PDF is loaded into the iframe, the options in the selectmenu go behind the iframe/pdf when dropped-down.  When the iframe is loaded with plain HTML, the selectmenu items are displayed correctly over the iframe when dropped down.  I'm having this issue on IE 9 and IE 11.  I've tested on Chrome and Firefox, and they both work correctly.  Any ideas?  Thanks!