By default, screen does not allow scrolling with regular PgUp/PgDn or
arrow keys. However, there is a way to move around: the is a copy mode.
This mode can be entered by pressing the regular prefix combination (^A), and
then hitting Esc. Moving around is possible with arrow keys. To get back to
regular mode hit Esc twice.
Source at Unix Stack Exchange.
Even better option is to put following into .screenrc:
termcapinfo xterm* ti@:te@This will enable regular scrolling with mouse wheel or terminal scrollbar.