devdocs-8.x-1.x-dev/devdocs_export/devdocs_export.module

<?php

/**
 * @file
 * Contains devdocs.module.
 */

use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Render\Element;
use Drupal\Core\Url;
use Drupal\Core\Link;

/**
 * Implements hook_help().
 */
function devdocs_export_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    // Main module help for the devdocs module.
    case 'help.page.devdocs_export':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('Description') . '</p>';
      return $output;

    default:
  }
}

/**
 * Implements hook_form_FORM_ID_alter().
 */
function devdocs_export_form_devdocs_files_form_alter(&$form, &$form_state, $form_id) {
  if (isset($form['files']) && is_array($form['files'])) {
    foreach (Element::children($form['files']) as $key) {

      $element = &$form['files'][$key];
      $title = $element['#title'];
      $uri = parse_url($element['htabs']['edit_' . $title]['uri_' . $title]['#value'], PHP_URL_HOST);
      $uri .= parse_url($element['htabs']['edit_' . $title]['uri_' . $title]['#value'], PHP_URL_PATH);

      $url = file_create_url('docs/export/pdf');
      $url = Url::fromUri($url);
      $link_options = [
        'query' => [
          'file' => $uri,
        ],
        'attributes' => [
          'class' => 'button',
        ],
      ];
      $url->setOptions($link_options);
      $link = Link::fromTextAndUrl(t('PDF'), $url)->toString();
      $element['htabs']['htabs_output']['export_pdf'] = [
        '#markup' => $link,
        '#group' => 'htabs',
        '#weight' => '-10',
      ];
    }
  }
}