Explain how to access ViewState value of this page in the next page.

PreviousPage property is set to the page property of the nest page to access the viewstate value of the page in the next page. Page poster = this.PreviousPage;
Once that is done, a control can be found from the previous page and its state can be read.
Label posterLabel = poster.findControl("myLabel");
string lbl = posterLabel.Text;