try this.
object oIsVisible=true;
mw.Document wrdDoc = wrdDocs.Open(ref wdFile,ref oMissing,ref oMissing,ref oMissing,ref oMissing,ref oMissing,ref oMissing,ref oMissing,ref oMissing,ref oMissing,ref oMissing,ref oMissing,ref oMissing,ref oMissing,ref oMissing,ref oIsVisible);
wrdDoc .activate();