try this way , this works for me
Microsoft.Office.Interop.Word.ApplicationClass ac = new Microsoft.Office.Interop.Word.ApplicationClass();
Microsoft.Office.Interop.Word.Application app = ac.Application;
app.DisplayAlerts = Microsoft.Office.Interop.Word.
WdAlertLevel.wdAlertsNone;
object filename = strFileName;
object missingValue = Type.Missing;
app.Visible = true;
Microsoft.Office.Interop.Word.
Document document = app.Documents.OpenOld(ref filename,ref missingValue, ref missingValue,ref missingValue, ref missingValue, ref missingValue,ref missingValue, ref missingValue, ref missingValue, ref missingValue);
app.Activate();
}