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Use SendMessage function and WM_VSCROLL message with SB_PAGEDOWN or SB_PAGEUP parameters.

procedure TForm1.Button2Click(Sender: TObject);
begin
  SendMessage(Memo1.Handle, WM_VSCROLL, SB_PAGEUP, 0);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  SendMessage(Memo1.Handle, WM_VSCROLL, SB_PAGEDOWN, 0);
end;