In the session pass the file path to ViewPage.aspx. In page Viewpage.aspx in your !Postback method load the session value and use the following code:
WebClient client = new WebClient();
Byte[] buffer = client.DownloadData(filepath);
Response.ContentType = "application/msword";
Response.AddHeader("content-length", buffer.Length.ToString());
Response.BinaryWrite(buffer);
Good luck,
OI