I use a memory stream and pass it to the WordprocessingDocument.Open
method. After I'm done changing the document, I just write the bytes to the destination:
var source = File.ReadAllBytes(filename);
using (var ms = new MemoryStream()) {
ms.Write(source, 0, source.Length);
/* settings defined elsewhere */
using (var doc = WordprocessingDocument.Open(ms, true, settings)) {
/* do something to the doc */
}
/* used in File.WriteAllBytes elsewhere */
return ms.ToArray();
}