Override entity_print filename on Drupal 8/9.

Often times clients want to change the PDF document filename created by entity_print module.

One of the ways to make this happens is to create a service extending ServiceProviderBase and alter the class that will process this filename.

For that, create a new file at the /src folder on your custom module and extend the ServiceProviderBase. Ex: /cms_module/src/CmsServiceProvider.php

<?php
namespace Drupal\cms_module;
use Drupal\Core\DependencyInjection\ContainerBuilder;
use Drupal\Core\DependencyInjection\ServiceProviderBase;
use Symfony\Component\DependencyInjection\Reference;
/**
* Modifies the entity_print FilenameGenerator service.
*/
class CmsServiceProvider extends ServiceProviderBase {
  /**
   * {@inheritdoc}
   */
  public function alter(ContainerBuilder $container) {
    $definition = $container
      ->getDefinition('entity_print.filename_generator');
    $definition->setClass('Drupal\cms_module\GenerateFilename')
      ->addArgument(new Reference('transliteration'));
  }
}

In my case, I’m calling the class GenerateFilename to process the filename for me. So on /cms_module/src/GenerateFilename.php we have:

<?php
namespace Drupal\cms_module;
use Drupal\Component\Transliteration\TransliterationInterface;
use Drupal\entity_print\FilenameGeneratorInterface;
/**
 * A service override for generating filenames for printed documents.
*/
class GenerateFilename implements FilenameGeneratorInterface {
 /**
  * The transliteration service.
  *
  * @var \Drupal\Component\Transliteration\TransliterationInterface
  */
 protected $transliteration;
 /**
  * FilenameGenerator constructor.
  *
  * @param \Drupal\Component\Transliteration\TransliterationInterface $transliteration
  *   Transliteration service.
  */
 public function __construct(TransliterationInterface $transliteration) {
  $this->transliteration = $transliteration;
 }
/**
 * {@inheritdoc}
 */
 public function generateFilename(array $entities, callable $entity_label_callback = NULL) {
  $filenames = [];
  
  /** @var \Drupal\Core\Entity\EntityInterface $entity */
  foreach ($entities as $entity) {
    if ($label = trim($this->sanitizeFilename($entity_label_callback ? $entity_label_callback($entity) : $entity->label(), $entity->language()->getId()))) {
      $filenames[] = $label;
    }
  }
 return $filenames ? implode('-', $filenames) : static::DEFAULT_FILENAME;
}
/**
 * Gets a safe filename.
 *
 * @param string $filename
 *   The un-processed filename.
 * @param string $langcode
 *   The language of the filename.
 *
 * @return string
 *   The filename stripped to only safe characters.
 */
protected function sanitizeFilename($filename, $langcode) {
  $transformed = $this->transliteration->transliterate($filename, $langcode);
  $clean = preg_replace("/[^A-Za-z0-9 ]/", '', $transformed);
  $clean = strtolower(str_replace(" ", "_", $clean));
  return $clean;
 }
}

Medium needs to improve a lot to be bad, but I hope this can give you some idea on how to implement these methods to override the PDF filename.