ext_localconf.php

<?php
if (!defined('TYPO3_MODE')) {
	die('Access denied.');
}

// Unserializing the configuration so we can use it here
$_EXTCONF = unserialize($_EXTCONF);
/** @var string $_EXTKEY The current extension key. */
$GLOBALS['TYPO3_CONF_VARS']['EXTCONF'][$_EXTKEY]['cacheLifetime'] = isset($_EXTCONF['cacheLifetime']) ? intval($_EXTCONF['cacheLifetime']) : 2592000;
$GLOBALS['TYPO3_CONF_VARS']['EXTCONF'][$_EXTKEY]['cacheLimit'] = isset($_EXTCONF['cacheLimit']) ? intval($_EXTCONF['cacheLimit']) : 0;
$GLOBALS['TYPO3_CONF_VARS']['EXTCONF'][$_EXTKEY]['debug'] = isset($_EXTCONF['debug']) ? boolval($_EXTCONF['debug']) : FALSE;
$GLOBALS['TYPO3_CONF_VARS']['EXTCONF'][$_EXTKEY]['pdfCacheFolderIdentifier'] = isset($_EXTCONF['pdfCacheFolderIdentifier']) ? $_EXTCONF['pdfCacheFolderIdentifier'] : '0:/typo3temp/tx_webkitpdf/';

$GLOBALS['TYPO3_CONF_VARS']['SYS']['caching']['cacheConfigurations'][\Tx\Webkitpdf\Utility\CacheManager::CACHE_IDENTIFIER] = array(
	'frontend' => \TYPO3\CMS\Core\Cache\Frontend\StringFrontend::class,
	'backend' => \Tx\Webkitpdf\Utility\CacheDatabaseBackend::class,
	'options' => array(
		'defaultLifetime' => $GLOBALS['TYPO3_CONF_VARS']['EXTCONF'][$_EXTKEY]['cacheLifetime'],
		'maximumNumberOfEntries' => $GLOBALS['TYPO3_CONF_VARS']['EXTCONF'][$_EXTKEY]['cacheLimit'],
	),
	'groups' => array('pages', 'all')
);