manipulating embed src tag in javascript

Question

i want to handle embed tag in java script here is my code but the pdf doesnot appear and i don't know why

function showbook(Object) {
   switch(Object.id) {
     case "a":
       document.getElementById("em").src="majorSins_2.pdf";
       break;
  } 
}

and in html document

<embed id="em" src="" width=600 height=600 border="2"/>