Hi experts,
I was able to solve it myself simply adding a javascript code under the button on_client_click value inside GET_BUTTONS method.
Although there is sth I had to watch out for. The javascript code, which opened another window (or tab) was triggered at the same time (or few miliseconds earlier) as the event handler for button press. Therefore I wasn't able to get current values of all fields used in the printed preview.
To solve it I had to add the setTimeout function to postpone for 500 miliseconds the javascript method being triggered. Of course I couldn't desactivate the event handler because it was responsible for the round trip, which caused that all values were up to date
Best regards,
Robert