There is no straight-forward way of doing this as far as I know. The Word interop libs are not able to read from a byte stream. Unless you are working with huge (or a huge amount of) files, I would recommend simply using a tmp file:
Application app = new Application();
byte[] wordContent = GetBytesInSomeWay();
var tmpFile = Path.GetTempFileName();
var tmpFileStream = File.OpenWrite(tmpFile);
tmpFileStream.Write(wordContent, 0, wordContent.Length);
tmpFileStream.Close();
app.Documents.Open(tmpFile);
I know this isn't the answer you're looking for, but in a case like this (where doing what you really want to do requires quite a bit of time and fidgeting) it might be worth considering whether or not development time outweighs runtime performance.
If you still want to look into a way to solve it the way you intend it to, I'd recommend the answers in this thread.
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