pdf as background to canvas to draw on pdf

If i load the html page instead of pdf. it works fines. i.e html page in background of canvas.  i used following code

<?xml version="1.0" encoding="utf-8"?>
<mx:WindowedApplication xmlns:mx="http://www.adobe.com/2006/mxml"layout="absolute">
    <mx:Script>
        <![CDATA[
            import mx.containers.Canvas;
           
            private var drawing:Boolean = false;
           
            private function canvasInit(e:Event):void {
                var c:Canvas = Canvas(e.target);
                c.graphics.lineStyle(2, 0xff0000, 1);
               
                c.addEventListener(MouseEvent.MOUSE_UP, onMouseEvent);
                c.addEventListener(MouseEvent.MOUSE_DOWN, onMouseEvent);
                c.addEventListener(MouseEvent.MOUSE_MOVE, onMouseEvent);
            }
           
            private function onMouseEvent(e:MouseEvent):void {
                //trace(e.type);
                if (e.type == MouseEvent.MOUSE_DOWN)
                    drawing = true;
                else if (e.type == MouseEvent.MOUSE_UP)
                    drawing = false;
                else if (e.type == MouseEvent.MOUSE_MOVE) {
                    var c:Canvas = Canvas(e.target);
                   
                    //trace((drawing ? "drawing to " : "moving to ") + e.localX + "," + e.localY);
                    if (drawing)
                        c.graphics.lineTo(e.localX, e.localY);
                    else
                        c.graphics.moveTo(e.localX, e.localY);
                }
            }
        ]]>
    </mx:Script>
    <mx:HTML top="0" bottom="0" left="0" right="0" location="http://labs.adobe.com/technologies/afcs/"/>
    <mx:Canvas top="0" bottom="0" left="0" right="0" initialize="canvasInit(event)">
    </mx:Canvas>
</mx:WindowedApplication>

instead of html page, if i put any pdf  file. it is not in background. it comes to the foreground. i don't know how to set pdf as background. please help me