code: protected function prepareForSave($pFilename)
      {
          //  garbage collect
          $this->spreadsheet->garbageCollect();
  
          $this->saveArrayReturnType = Calculation::getArrayReturnType();
          Calculation::setArrayReturnType(Calculation::RETURN_ARRAY_AS_VALUE);
  
          //  Open file
          $fileHandle = fopen($pFilename, 'w');
          if ($fileHandle === false) {
              throw new WriterException("Could not open file $pFilename for writing.");
          }
  
          //  Set PDF
          $this->isPdf = true;
          //  Build CSS
          $this->buildCSS(true);
  
          return $fileHandle;
      }
docs: Save Spreadsheet to PDF file, pre-save.
  
  @param string $pFilename Name of the file to save as
  
  @throws WriterException
  
  @return resource