phpでファイルを出力できない場合はどうすればよいですか?

require_once('../tcpdf/tcpdf.php');
require_once('../FPDI/fpdi.php');
$fileName = '../sample.pdf';
class PDF extends FPDI {
/**
 * "Remembers" the template id of the imported page
 */
var $_tplIdx;
var $numPages = 0;
/**
 * Draw an imported PDF logo on every page
 */
function Header() {
    global $fileName;
    if (is_null($this->_tplIdx)) {
        $this->setSourceFile($fileName);
        $this->_tplIdx = $this->importPage(1);
        $this->numPages = $this->setSourceFile($fileName);
    }
    $size = $this->useTemplate($this->_tplIdx);
}
function Footer() {
    // emtpy method body
}
}
// initiate PDF
$pdf = new PDF($fileName);
$pdf->setFontSubsetting(true);
// add a page
$pdf->AddPage();
// save file
$pdf->Output('output.pdf', 'F');

ob_clean();
// save file
$pdf->Output('output.pdf', 'F');

$pdf->Output('yourpath/output.pdf', 'F');

$pdf->Output($_SERVER['DOCUMENT_ROOT'] . 'output.pdf', 'F');