I've installed multitasking and played around with it for some days. This tool is really great! Thank you Booxtor
I've found out a bug:
when I choose to use an old pdf reader and open, e.i. 2, pdfs than I can't close them in anyway besides restarting device. when I choose one of such pdf and press menu button (house button) than I see settings tag, so device think that pdf reader is closed, but it isn't.