Implementing Custom Pagination without Drupal Entity Query - Drupal 8

DrupalCallback.php

<?php

namespace Drupal\custom_module\Controller;

use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\Core\Pager\PagerManagerInterface;

class ListNonDrupalItems extends ControllerBase {

  /**
   * Drupal\Core\Pager\PagerManagerInterface definition.
   *
   * @var \Drupal\Core\Pager\PagerManagerInterface
   */
  protected $pagerManager;
  
  public function __construct(PagerManagerInterface $pagerManager) {
    $this->pagerManager = $pagerManager;
  }
  
  public static function create(ContainerInterface $container) {
    $instance = $container->get('pager.manager');
    return new static($instance);
  }
  
  public function build() {
    $build = [];
    
    // Get data.
    $items = $this->getBigListData();
    $total = count($items);
    $limit = 10;
    
    // Initialize pager and get current page.
    $pager = $this->pagerManager->createPager($totalFiles, $limit);
    $currentPage = $pager->getCurrentPage();
    
    // Use currentPage to limit items for the page.
    $items = array_slice($items, $currentPage * $limit, $limit);
    
    $build['list'] = [
      '#theme' => 'item_list',
      '#list_type' => 'ol',
      '#title' => 'List',
      '#items' => [],
    ];
    
    // Display items.
    foreach ($items as $item) {
      $build['list']['#items'][] = [
        '#wrapper_attributes' => [
          'class' => ['item'],
        ],
        '#children' => $item,
      ];
    }
    
    $build['pager'] = [
      '#type' => 'pager',
    ];
    
    return $build;
  }
  
  private function getBigListData() {
    return ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'];
  }

}

there is one mistake with variable name $totalFile should rename with $total.