Use an invisible <iframe>:
<iframe id="my_iframe" style="display:none;"></iframe>
<script>
function Download(url) {
document.getElementById('my_iframe').src = url;
};
</script>
To force the browser to download a file it would otherwise be capable of rendering (such as HTML or text files), you need the server to set the file’s MIME Type to a nonsensical value, such as application/x-please-download-me or alternatively application/octet-stream, which is used for arbitrary binary data.
If you only want to open it in a new tab, the only way to do this is for the user to a click on a link with its target attribute set to _blank.
In jQuery:
$('a#someID').attr({target: '_blank',
href : 'http://localhost/directory/file.pdf'});
Whenever that link is clicked, it will download the file in a new tab/window.