data.cs

[System.Web.Http.HttpGet]
public HttpResponseMessage LoadDocument(string ConnectionID, string DocumentName)
{
    // connect the WebSocketHandler with the ConnectionID
    WebSocketHandler wsHandler = WebSocketHandler.GetInstance(ConnectionID);

    // the document directly server-side
    wsHandler.LoadText(System.Web.Hosting.HostingEnvironment.MapPath("~/App_Data/" + DocumentName),
        StreamType.WordprocessingML);

    return new HttpResponseMessage()
    {
        StatusCode = HttpStatusCode.OK
    };
}