function pdfWindow opening blank page

This used to work and suddenly it does not work in IE.

Here's the function

function pdfWindow(loc)
{
if(loc == "#RefQuide") {
location.href = loc;
} else {
// The window attributes
var window_attribs = "height=650,width=850,resizable=yes,scrollbars=yes";

window.open(loc,"PDF",window_attribs);
}
}


Here's part of the HTML

<form name="myForm">
<select name="hypercomList">
<option value="/CWRWeb/pdf/HypercomT4000.pdf">Hypercom T4000 </option>
<option value="/CWRWeb/pdf/HypercomT4100.pdf">Hypercom T4100 </option>
</select>
<input type="button" value="Go!" onclick="pdfWindow(myForm.hypercomList.options[myForm.hypercomList.selectedIndex].value)">

</form>


Any ideas why it would open a blank page instead of the PDF? Works ok in FF.