PDFsharp & MigraDoc Foundation

I want to combine two PDF documents into one. I have both documents stored in a database, so I retrieve them into byte arrays (C#).

Here is the code I have so far (pdf1 and pdf2 are byte arrays):

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main
PdfDocument outputDocument = new PdfDocument();

AddPages(outputDocument, pdf1)
AddPages(outputDocument, pdf2)

*** now I want to do outputDocument.Save. but to a byte array, not a file ***
>

private void AddPages(PdfDocument outputDocument, byte[] pdfBytes)
MemoryStream ms = new MemoryStream(pdfBytes);
PdfDocument inDoc = PdfReader.Open(ms, PdfDocumentOpenMode.Import);
for (int idx = 0; idx < inDoc.PageCount; idx++)
// Get the page from the external document.
PdfPage page = inDoc.Pages[idx];
// . and add it to the output document.
outputDocument.AddPage(page);
>
>
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What is the syntax to use outputDocument.Save(stream) so I can end up with a byte array of the combined pdfs?