If you think there is an issue with PdfDocument.Save, then please report this on the PDFsharp forum (but please be more specific with your error description).
Your “solution” looks like a hack to me.
“pdfRenderer.Save” calls “PdfDocument.Save” internally.
Whatever the problem is – your “solution” still calls the same Save routine.
Edit:
To get a byte[] containing a PDF file, you only have to call:
MemoryStream stream = new MemoryStream();
document.Save(stream, false);
byte[] bytes = stream.ToArray();
Early versions of PDFsharp do not reset the stream position.
So you have to call
ms.Seek(0, SeekOrigin.Begin);
to reset the stream position before reading from the stream; this is no longer required for current versions.
Using ToArray can often be used instead of reading from the stream.
Edit 2: instead of stream.ToArray() it may be more efficient to use stream.GetBuffer(), but this buffer is usually larger than the PDF file and you only have to use stream.Length bytes from that buffer. Very useful for method that take a byte[] along with a length parameter.