如何保存fpdf文件的顶部和底部内容？

Question

我正在使用FPDF从PHP生成pdf文件。但我遇到的一个困难是，当文件很大时，它将覆盖页脚内容和页眉内容的下一页。我需要帮助。

class PDF extends FPDF {

// Page header
function Header() {
    // Logo
    $this->Image('custom/pics/logoo.png', 8, 15, 20);
    // Arial bold 15
    $this->SetFont('Arial', 'B', 15);
    // Move to the right
    $this->Cell(30);
    // Title
    $this->Cell(30, 20, 'Payment Request', 0, 0, 'C');
    // Line break
    $this->Ln(20);
}

// Page footer
function Footer() {
    // Position at 1.5 cm from bottom
    $this->SetY(-25);
    // Arial italic 8
    $this->SetFont('Arial', '', 6);
    // Page number
    $this->Cell(0, 3, 'Price Quoted: ETB including VAT', 0, 1, 'L');
    $this->Cell(0, 3, 'Delivery Period: Already installed and configured.', 0, 1, 'L');
    $this->Cell(0, 3, 'Remarks: Make all checks payable to Mella COMMUNICATION TECHNOLOGY PLC', 0, 1, 'L');
    $this->SetFont('Arial', 'I', 8);
    $this->Cell(0, 3, 'THANK YOU FOR YOUR BUSINESS!', 0, 0, 'C');
    $this->Cell(0, 3, 'Page ' . $this->PageNo() . '/{nb}', 0, 0, 'R');
}

}

// Instanciation of inherited class
$pdf = new PDF();
$pdf->AliasNbPages();
$pdf->AddPage();

$pdf->SetFillColor(176, 179, 177);

$pdf->SetFont('Arial', '', 8);
//Installed Vehicle list with details

if (isset($_POST["device_pdf"])) {
    $for = count($_POST["device_pdf"]);
    $item_no += 1;
    $pdf->SetFont('Arial', 'B', 8);
    $pdf->Cell(70, 7, $item_no . '. Device Purchase', 'L,R', 0, 'L');
    $pdf->Cell(25, 7, "", 'L,R', 0);
    $pdf->Cell(30, 7, "", 'L,R', 0);
    $pdf->Cell(25, 7, "", 'L,R', 0);
    $pdf->Cell(40, 7, "", 'L,R', 1);
    $pdf->SetFont('Arial', '', 7);           
    }
}

这只是整个代码的一个片段。