Example hook for how to use Unoconv service. https://drupal.org/project/unoconv_service

unoconv_example.module

<?php
/**
 * Implements hook_ENTITY_TYPE_presave().
 */
function unoconv_example_media_presave(EntityInterface $entity) {

  $original_file_id = $entity->get('field_document_file')->getValue();
  /** @var \Drupal\file\Entity\File $file */
  $file = File::load($original_file_id[0]['target_id']);

  // Get actual path.
  $actual_path = \Drupal::service('file_system')->realpath($file->getFileUri());

  // Prepare destination path.
  $file_directory_path = \Drupal::service('file_system')->realpath(file_default_scheme() . "://");
  $new_filename = pathinfo($file->getFilename(), PATHINFO_FILENAME) . '.pdf';
  $destination_path = $file_directory_path . '/preview/' . $new_filename;
  $destination_uri = file_default_scheme() . '://preview/' . $new_filename;

  $transcode_service = \Drupal::service('unoconv_service.transcode');
  $preview_file = $transcode_service->transcode($actual_path, $destination_path, $destination_uri);

  $entity->set('field_document_media_preview1', ['target_id' => $preview_file->id()]);
}