Copy and Paste the below code. it will work for you but make sure you are pointing out the jquery file in the HEAD section.
<script type="text/javascript">
function showpf(){
if ($('#<%=chk_pf.ClientID %>').is(':checked')) {
$(".pf").show();
}
else {
$(".pf").hide();
}
}
</script>
You just use Uppercase C in ":Checked" which should be Lowercase ":checked"
Hope it helps :)