code: public function save($pFilename)
      {
          $fileHandle = parent::prepareForSave($pFilename);
  
          //  Default PDF paper size
          $paperSize = 'LETTER'; //    Letter    (8.5 in. by 11 in.)
  
          //  Check for paper size and page orientation
          if ($this->getSheetIndex() === null) {
              $orientation = ($this->spreadsheet->getSheet(0)->getPageSetup()->getOrientation()
                  == PageSetup::ORIENTATION_LANDSCAPE) ? 'L' : 'P';
              $printPaperSize = $this->spreadsheet->getSheet(0)->getPageSetup()->getPaperSize();
          } else {
              $orientation = ($this->spreadsheet->getSheet($this->getSheetIndex())->getPageSetup()->getOrientation()
                  == PageSetup::ORIENTATION_LANDSCAPE) ? 'L' : 'P';
              $printPaperSize = $this->spreadsheet->getSheet($this->getSheetIndex())->getPageSetup()->getPaperSize();
          }
  
          $orientation = ($orientation == 'L') ? 'landscape' : 'portrait';
  
          //  Override Page Orientation
          if ($this->getOrientation() !== null) {
              $orientation = ($this->getOrientation() == PageSetup::ORIENTATION_DEFAULT)
                  ? PageSetup::ORIENTATION_PORTRAIT
                  : $this->getOrientation();
          }
          //  Override Paper Size
          if ($this->getPaperSize() !== null) {
              $printPaperSize = $this->getPaperSize();
          }
  
          if (isset(self::$paperSizes[$printPaperSize])) {
              $paperSize = self::$paperSizes[$printPaperSize];
          }
  
          //  Create PDF
          $pdf = $this->createExternalWriterInstance();
          $pdf->setPaper(strtolower($paperSize), $orientation);
  
          $pdf->loadHtml(
              $this->generateHTMLHeader(false) .
              $this->generateSheetData() .
              $this->generateHTMLFooter()
          );
          $pdf->render();
  
          //  Write to file
          fwrite($fileHandle, $pdf->output());
  
          parent::restoreStateAfterSave($fileHandle);
      }
docs: Save Spreadsheet to file.
  
  @param string $pFilename Name of the file to save as
  
  @throws \PhpOffice\PhpSpreadsheet\Writer\Exception