Google custom search กำหนดให้ค้นหาเฉพาะไฟล์ PDF ยังไงคะ

 (function() {
  var cx = 'xxxxxxxxxxxxxxxx:yyyyyyyyyyyyyyyyyyy'; // Insert your own Custom Search Engine ID here
  var gcse = document.createElement('script'); gcse.type = 'text/javascript'; gcse.async = true;
  gcse.src = (document.location.protocol == 'https' ? 'https:' : 'http:') +
      '//www.google.com/cse/cse.js?cx=' + cx;
  var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(gcse, s);
  
  customSearchControl.setSearchStartingCallback(
    this,
    function(control, searcher, query) {
      searcher.setQueryAddition('filetype:pdf');
    }
  );

})();

 <gcse:searchbox></gcse:searchbox>

 <gcse:searchresults></gcse:searchresults>