Scroll form by PgDn and PgUp

You should set KeyPreview property of the Form to True and use VertScrollBar property of your form to change position of the form.

procedure TForm1.FormKeyDown(Sender: TObject; var Key: Word;
  Shift: TShiftState);
begin
  if Key=VK_NEXT then
    VertScrollBar.Position:=
      VertScrollBar.Position+
      StrToInt(Edit1.Text)
  else
    VertScrollBar.Position:=
      VertScrollBar.Position-
      StrToInt(Edit1.Text);
end;