I see that you are originally loading MooTools and then loading jQuery. Is there a reason that you need both?
I would guess that since $ is loaded first then it is referencing MooTools and not jQuery, but I could be mistaken. I have never needed both libraries on the same page.
For a Mootools solution try something like:
Code:
<script type="text/javascript">
$$("a[href$='pdf']").each(function(pdfLink) {
pdfLink.addEvent('click', function(e) {
_gaq.push(['_trackEvent', 'PDF', 'Download', pdfLink.href]);
});
});
</script>
This is mootools syntax to select the 'a' tags linking to a pdf file. Loops through the results. Adds a click event for each link. That click event will add the new event to the google analytics array.
Good Luck