you can't do that as you are making ajax call to server and for downloading something from server you have to make complete postback.
So you can do like this.
in your ajax call make or save your pdf file on the server like YourApplicationName/PDF/sample.pdf.
and return this path from your ajax call.Now your file is on the server and you know the path but the thing that is remaining is download this file.For this you can use complete function and hidden iframe complete:function(){
//your code
var iframe = document.createElement("iframe");
iframe.src = "GenerateFile.aspx?page=ViewDrawing";
iframe.style.display = "none";
document.body.appendChild(iframe);
}
And in GenerateFile.aspx page load event you can write your pdf downloadable code.