Meta tags are not a reliable way to achieve this result. Generally you shouldn't even do this - it should be left up to the user/user agent to decide what do to with the content you provide. The user can always force their browser to download the file if they wish to.
If you still want to force the browser to download the file, modify the HTTP headers directly. Here's a PHP code example:
$path = "path/to/file.pdf";
$filename = "file.pdf";
header('Content-Transfer-Encoding: binary'); // For Gecko browsers mainly
header('Last-Modified: ' . gmdate('D, d M Y H:i:s', filemtime($path)) . ' GMT');
header('Accept-Ranges: bytes'); // Allow support for download resume
header('Content-Length: ' . filesize($path)); // File size
header('Content-Encoding: none');
header('Content-Type: application/pdf'); // Change the mime type if the file is not PDF
header('Content-Disposition: attachment; filename=' . $filename); // Make the browser display the Save As dialog
readfile($path); // This is necessary in order to get it to actually download the file, otherwise it will be 0Kb
Note that this is just an extension to the HTTP protocol; some browsers might ignore it anyway.