Wildfly: ManagedExecutorService and PipedOutputStream

I need help regarding the ManagedExecutorService (I´m using Wildfly).

I have in my JSF Bean (Controller) a method like this:

public void doSendEmail() {
    myEjbean.sendOffer(offer);
}

This will call my EJB function: I would like to create now a PDF:

    PipedOutputStream createPdfWithAttachments(DocumentDesigner documentDesigner, String fileName) {

 List<InputStream> inputStreamList = new ArrayList<InputStream>();

// 1) Add InputStream (which should be added before the document from 2)
// // Diese hinzufügen bei: inputStreamList.add(inputStream);

// 2) Create Document (from a URL)
PipedOutputStream createPdf(PipedOutputStream os, DocumentDesigner documentDesigner, boolean preview)
inputStreamList.add(isDocument);

// 3) Add InputStream (Stream which should be attach after the document)
// Diese hinzufügen bei: inputStreamList.add(inputStream);


// 4) Merge PDF 
PipedOutputStream is = pdfGenerationHelperService.mergePdf(inputStreamList, fileName);
return is;

}

In another function, I´m calling this function to create the InputStream and attach that to an object for my Email Attachment:

PipedOutputStream os = new PipedOutputStream();
             PipedInputStream is = new PipedInputStream(os);
             os = pdfDocumentGenerationService.createPdfWithAttachments(offer.getDocumentDesigner(),
                    fileName, false);

            EmailAttachment emailAttachment = new EmailAttachment();
            emailAttachment.setInputStream(is);
            emailAttachment.setFilename(fileName);
            emailAttachment.setMimeType("application/pdf");

I´m currently struggeling when I should use this:

 managedExecutorService.submit(() -> {
        try {
            doSomething();
        }
    catch (Exception e) {}
        return null;
    });

It seems inside the createPdf - function I need this managedExecutorService.submit(() -> {...} otherwise it will not work. Here the code for this function:

    /**
 * Create PDF
 * 
 * @throws IOException
 */
public PipedOutputStream createPdf(PipedOutputStream os, DocumentDesigner documentDesigner, boolean preview)
        throws IOException {

    managedExecutorService.submit(() -> {

        try {
            String url = generateUrl(documentDesigner, preview);

            PdfRendererBuilder builder = new PdfRendererBuilder();
            XRLog.setLevel(XRLog.CSS_PARSE, Level.SEVERE);
            XRLog.setLoggingEnabled(false);

            // builder.withUri(new URL(url).toString());
            Document doc = html5ParseDocument(url, 0);
            builder.withW3cDocument(doc, url);

            builder.toStream(os);
            builder.run();

            os.close();
            return os;
        }

        catch (Exception e) {
            LOGGER.error(ExceptionUtils.getFullStackTrace(e));
        }

        return null;
    });

    return null;
}

I would like to know when I have to use managedExecutorService.submit(() -> and can I have nested managedExecutorService.submit(() ?

In general what I´m trying to do here:

1. Send an email with a "Offer" - Object
2. Offer will be generated as PDF
3. PDF will merge other PDF´s to it
4. Attach the PDF to EmailAttachment
5. Send Email

Many thanks for help