Most PDA connects to PC USB, as client. I'm yet to find PDA with USB Host capability. Even if you can find CF-to-parallel adaptor, new software is needed. IMHO, your best bet would be:-
1. Get
CNC controller with serial interface (DeskCNC or similar).
2. Get/use serial interface for your PDA.
3. Talk/email to the controller manufacturer on the communication protocol.
4. Write simple comm program to send the G-Code to the controller.
If size is your main concern, get an industrial PC with LCD screen. It should be not much larger than you PDA with all the addon.