os.system("helpfile.pdf") goes to next line when file is open. It doesn't wait until user close it. So helpfile_btn is deactive only for a moment because the next line makes it working again. I don't think that it's possible to do with reader that select in system. In windows you can't get access to reader. And almost you don't know to witch. Acrobate? Chrome? Firefox... Maby don't do it or make reader a part of your программе?