function sendCsvFile( data )
{
    var blob=new Blob([data]);
    var link=document.createElement('a');
    link.href = window.URL.createObjectURL(blob);
    link.download = "export_file.csv";
    link.click();
}

    $(document).ready(function(){
        $( '#downloadBtn' ).click( function() {
            $.ajax( {
                "type": 'POST',
                "data": $('#formular').serializeArray(),
                "url": 'ajax.php&action=download',
                success: function(data) {
                    sendCsvFile( data );
                }
            });
        } )
    });