PHP - Fatal error: Call to undefined method PDF::Open()

<?php

define('FPDF_FONTPATH','fpdf/font/');

include('PDF_MC_Table.php');

include("conectaBD.php");

$con1=conectar();

$anio=$_POST['anio'];

$consulta1="select * from dotacion";

$result1 = mysql_query($consulta1,$con1);

if($row1 = mysql_fetch_array($result1))

{

    class PDF extends PDF_MC_Table

    {

        function Header()

        {

            $this->SetFont('times','',9);

            $pdf=new PDF('L','mm','A4');

            $pdf->Open();  (AQUI ES LA LINEA 36)

            $pdf->AddPage();

            //Mostrar todas las columnas automaticamente

            $pdf->SetFont('times','B',12);

            $pdf->Cell(0,4,'REPORTE DE DOTACION',8,30,'C');

            $pdf->SetFont('times','B',10);