function newWindow() {
 if (!document.getElementsByTagName) return;

 var anchors = document.getElementsByTagName("a");

 for (var i=0; i<anchors.length; i++) {
   var anchor = anchors[i];
   if (anchor.getAttribute("href") &&
       anchor.getAttribute("rel") == "pdf")
     anchor.target = "_blank";
 }
}

window.onload = newWindow;