pdf下载 html2pdf

我正在使用 html2pdf 类来生成 pdf。在我的问题中，它为 html 代码生成 pdf，但它没有提供下载该 pdf 的对话框选项。请帮助我的情况如下。

<?php
ob_start();
include(dirname(__FILE__).'/res/pdf_demo.php');
$content = ob_get_clean();

// conversion HTML => PDF
require_once(dirname(__FILE__).'/../html2pdf.class.php');
try
{
    $html2pdf = new HTML2PDF('P','A4', 'fr', false, 'ISO-8859-15');
    $html2pdf->pdf->SetDisplayMode('fullpage');
    $html2pdf->writeHTML($content, isset($_GET['vuehtml']));
    $html2pdf->Output('pdf_demo.pdf'); 
}
catch(HTML2PDF_exception $e) { echo $e; }
?>

从文档中，方法输出

    /**
     * Send the document to a given destination: string, local file or browser.
     * Dest can be :
     *  I : send the file inline to the browser (default). The plug-in is used if available. The name given by name is used when one selects the "Save as" option on the link generating the PDF.
     *  D : send to the browser and force a file download with the name given by name.
     *  F : save to a local server file with the name given by name.
     *  S : return the document as a string. name is ignored.
     *  FI: equivalent to F + I option
     *  FD: equivalent to F + D option
     *  true  => I
     *  false => S
     *