It's not easy to count something from the welcome screen but if you hide that and place the welcome message in the first survey group, you can use JavaScript to load a hidden short-text question with the clicks.
1) Run the survey in group-by-group mode.
2) Place your welcome message in a text-display question in the first survey group.
3) Add a class "
count-clicks" to the <a> tag of the link to the PDF.
Code:
<a href="pathTo/PDF" target="_blank" class="count-clicks">Link to PDF</a>
4) Add a short-text to that group and give it a CSS class "
hidden".
5) Add this script to the question source of that question:
Code:
<script type="text/javascript" charset="utf-8">
$(document).ready(function(){
$('a.count-clicks').on('click', function(e) {
$('#question{QID} input[type="text"]').val(Number($('#question{QID} input[type="text"]').val())+1);
});
});
</script>