Use terminal42/contao-notification_center to send messages when using markocupic/calendar-event-booking-bundle. Store these two files in src/EventListener respectively in contao/config. The Hook will transform a file uuid to a file path. Thus the file will be attached to the notification.

config.php

<?php

// contao/config/config.php

// notification center
if (isset($GLOBALS['NOTIFICATION_CENTER']['NOTIFICATION_TYPE']['calendar-event-booking-bundle'])) {
    $GLOBALS['NOTIFICATION_CENTER']['NOTIFICATION_TYPE']['calendar-event-booking-bundle']['booking-notification']['attachment_tokens'] = ['document'];
}

SendNotificationMessage.php

<?php
// src/EventListener/SendNotificationMessage.php

declare(strict_types=1);

namespace App\EventListener;

use Contao\CoreBundle\ServiceAnnotation\Hook;
use Contao\FilesModel;
use NotificationCenter\Model\Message;
use NotificationCenter\Model\Gateway;

/**
 * @Hook("sendNotificationMessage")
 */
class SendNotificationMessage
{
    
    public function __invoke(Message $objMessage, array &$arrTokens, ?string $language, Gateway $objGatewayModel)
    {
        $intMessageId = 2; // assuming, that the id of your NC message is 2.

        if ((int)$objMessage->id === $intMessageId) {

            $objFiles = FilesModel::findByUuid($arrTokens['document']);

            if (null !== $objFiles) {
                // Convert uuid to file path relative to the document root.
                $arrTokens['document'] = $objFiles->path;
            }

            return true;
        }

        return false;
    }
  
}