Pimcore\Helper\Mail::setAbsolutePaths PHP Method

    public static function setAbsolutePaths($string, $document = null, $hostUrl = null)
    {
        if ($document && $document instanceof Model\Document == false) {
            throw new \Exception('$document has to be an instance of Document');
        }
        $replacePrefix = "";
        if (!$hostUrl && $document) {
            // try to determine if the newsletter is within a site
            $site = \Pimcore\Tool\Frontend::getSiteForDocument($document);
            if ($site) {
                $hostUrl = "http://" . $site->getMainDomain();
                $replacePrefix = $site->getRootPath();
            }
            // fallback
            if (!$hostUrl) {
                $hostUrl = \Pimcore\Tool::getHostUrl();
            }
        }
        //matches all links
        preg_match_all("@(href|src)\\s*=[\"']([^(http|mailto|javascript|data:|#)].*?(css|jpe?g|gif|png)?)[\"']@is", $string, $matches);
        if (!empty($matches[0])) {
            foreach ($matches[0] as $key => $value) {
                $path = $matches[2][$key];
                if (strpos($path, '//') === 0) {
                    $absolutePath = "http:" . $path;
                } elseif (strpos($path, '/') === 0) {
                    $absolutePath = preg_replace("@^" . $replacePrefix . "/@", "/", $path);
                    $absolutePath = $hostUrl . $absolutePath;
                } else {
                    $absolutePath = $hostUrl . "/{$path}";
                    $netUrl = new \Net_URL2($absolutePath);
                    $absolutePath = $netUrl->getNormalizedURL();
                }
                $path = preg_quote($path);
                $string = preg_replace("!([\"']){$path}([\"'])!is", "\\1" . $absolutePath . "\\2", $string);
            }
        }
        preg_match_all("@srcset\\s*=[\"'](.*?)[\"']@is", $string, $matches);
        foreach ((array) $matches[1] as $i => $value) {
            $parts = explode(',', $value);
            foreach ($parts as $key => $v) {
                $parts[$key] = $hostUrl . trim($v);
            }
            $s = ' srcset="' . implode(', ', $parts) . '" ';
            if ($matches[0][$i]) {
                $string = str_replace($matches[0][$i], $s, $string);
            }
        }
        return $string;
    }

 protected function buildPdf(Document\PrintAbstract $document, $config)
 {
     $web2printConfig = Config::getWeb2PrintConfig();
     $params = [];
     $this->updateStatus($document->getId(), 10, "start_html_rendering");
     $html = $document->renderDocument($params);
     $placeholder = new \Pimcore\Placeholder();
     $html = $placeholder->replacePlaceholders($html);
     $html = \Pimcore\Helper\Mail::setAbsolutePaths($html, $document, $web2printConfig->wkhtml2pdfHostname);
     $this->updateStatus($document->getId(), 40, "finished_html_rendering");
     file_put_contents(PIMCORE_TEMPORARY_DIRECTORY . DIRECTORY_SEPARATOR . "wkhtmltorpdf-input.html", $html);
     $this->updateStatus($document->getId(), 45, "saved_html_file");
     try {
         $this->updateStatus($document->getId(), 50, "pdf_conversion");
         $pdf = $this->fromStringToStream($html);
         $this->updateStatus($document->getId(), 100, "saving_pdf_document");
     } catch (\Exception $e) {
         Logger::error($e);
         $document->setLastGenerateMessage($e->getMessage());
         throw new \Exception("Error during REST-Request:" . $e->getMessage());
     }
     $document->setLastGenerateMessage("");
     return $pdf;
 }