[Old Pedant]

If you are not using jQuery for other things on the page, it's not worth dragging in that whole library for something as simple as this.

// use AJAX to start creation of the new PDF
var http = window.XMLHttpRequest != null 
            ? new XMLHttpRequest() 
            : new ActiveXObject("Microsoft.XMLHTTP");
http.open("GET","index.php?module=Patient_Procedure&action=genMFGPO'&pomode=create&kindofpo=Z",true);
http.send( ); 
// ignore the asynch response

// then just change the current popup to the new location:
location.href = "index.php?module=Patient_Procedure&action=genMFGPO&pomode=preview";