Why Document of Webbrowser is Nil?

If my computer only has PPView97 installed(not MS Office),the Document object
of Webbrowser is Nil? Why?

Can anybody help me,how to get its Document object, thanks a lot!

delphi code:
procedure TForm1.Button1Click(Sender: TObject);
begin
wb.Navigate('c:\1.ppt'); //wb is a TWebbrowser
if wb.Document=nil then showmessage('nil!');
end;