the onchange event may be causing the issue:
<select id="pdfSubCatID" name="pdfSubCatID" onchange="pdfCatID_reload(this)">
change that to:
<select id="pdfSubCatID" name="pdfSubCatID">
If your still having issues, I'll need to see it in the browser, let me know and I'll start a private ticket.