Powershell screen buffer
The scrollback is provided by Windows console itself, not by the shell inside (just like xterm and similar apps in Unix). And to activate its "scroll" feature, use Alt+Space, E, L as John Rennie described. (You could try the Scroll Lock key, but I can't test it myself here.)
Press F7. Not the same as page up and page down but still very useful.
JR
Re Alexander's comment: oops, sorry I misunderstood your question. Irritatingly there isn't a simply keystroke to page up or down through the screen buffer. However if you use the comination alt-space, E L (i.e. select Edit/Scroll from the system menu) you'll find the arrows and page up/down scroll through the screen buffer. Press escape to exit the "scroll mode".
Maybe there is a hotkey to enter scroll mode, but I've been able to find it.
In Windows 10 Powershell use Ctrl+PgUp/PgDn to scroll by line.
In Windows 10 cmd use Ctrl+↓/↑ to scroll by line.