procedure TForm1.Button1Click(Sender: TObject);
var
  WndHandle: HWND;
  TextValue: string;
  TextLenght: Integer;
begin
  WndHandle := FindWindowHandleByTitle('Notepad');
  TextLenght := SendMessage(WndHandle, WM_GETTEXTLENGTH, 0, 0);
  if TextLenght > 0 then
  begin
    SetLength(TextValue, TextLenght);
    TextLenght := SendMessage(WndHandle, WM_GETTEXT, TextLenght + 1, LPARAM(PChar(TextValue)));
    SetLength(TextValue, TextLenght);
    Memo1.Text := TextValue;
  end;
end;