如何从ajax调用中打开pdf？

Question

这是我尝试通过ajax打开pdf，但显然有些不正确。

（代码片断）

function show_invoice() {
    $data = $this->input->get('info', TRUE);

    $user_id = $data['user_id'];
    if ($user_id != strval(intval($user_id))) {
        throw new exception('no id for user is set for creating pdf');
    }

    //Load pdf into browser
    $filename_pdf = 'pdf/invoice'  . $user_id . '.pdf';                      
    header("Content-Length: " . filesize ( $filename_pdf ) ); 
    header("Content-type: application/octet-stream"); 
    header("Content-disposition: attachment; filename=".basename($filename_pdf)); 
    header('Expires: 0');
    header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
    ob_clean();
    flush();

    //Must use readfile because the file isn't 
    //accesible through the browser through for example
    //http://domain/pdf/invoice_21.pdf                                     
    readfile($filename_pdf); 
}

js（code-snippet）

var info = {};
info.user_id = user_id;

//Send get request to show pdf
var showInvoice = $.ajax({
    type: 'GET',
    data: {info: info},
    url: CI.base_url + 'member/show_invoice', //CI.base_url is a CodeIgniter constant
    dataType: "json" //WHAT datatype to use here?
});

showInvoice.done(function(data) {  
    //do something      
});        

我也试过echo json_encode(readfile($filename_pdf));的东西（但我暂时不相信）

更新

我正在使用AJAX，因为我无法像<a href="domain/pdf/invoice_21.pdf">Show invoice</a>这样做，因为该文件无法直接在浏览器中访问。