Sample implementation for Transcode method.

UnoconvTranscode.php

/**
* {@inheritdoc}
*/
public function transcode($source_path, $destination_path, $destination_uri) {
 $unoconv_config = $this->configFactory->get('unoconv_service.unoconvconfig');
 try {
   $unoconv = Unoconv::create(array(
     'timeout' => $unoconv_config->get('timeout'),
     'unoconv.binaries' => $unoconv_config->get('binaries_path'),
   ));
   $unoconv->transcode($source_path, 'pdf', $destination_path);
 } catch (ExecutionFailureException $exception) {
     throw new RuntimeException(
       t('Unoconv failed to transcode file'), $exception->getCode(), $exception);
 }

 // Saving new converted preview pdf file and attach it to media entity.
 $data = file_get_contents($destination_path);
 $preview_file = file_save_data($data, $destination_uri, FILE_EXISTS_RENAME);

 return $preview_file;
}