<form id="pdfform" target="_blank">
    <input type="hidden" id="bill_id_pdf" name="bill_id_pdf">
</form>

<script>
    function exporttopdf_common(bill_id) {

        $("#bill_id_pdf").val(bill_id);

        document.forms['pdfform'].action = "bill_form_pdf_invoice.php?bill_id=" + bill_id;
        document.forms['pdfform'].method = "post";
        document.forms['pdfform'].submit();

    }
</script>