When you are finished using ISPF/PDF, exit it by pressing the RETURN PF key (PF 4), or by pressing the END PF key (PF 3) repeatedly until you see a menu panel with an option X to exit.

To exit that menu panel, enter x on its OPTION line and on the OPTION lines of other menu panels that follow. You then see the READY message.

If you want to end your TSO/E session, enter logoff, under the READY message.