ccd application circuit
"why they're using a hex inverter and not just one"


...because you need 3 (not just one; their basic function is buffering the signal and just inverting by the way) and they come in packages containing six.


  "why is the pnp transistor needed."


...because the CCD output signal is high impedance (cannot provide much current). The transistor circuit is an emitter follower that leaves voltage about the same (just adds about 0.6V but that doesn't matter at all in this application, because the signal will have a more or less arbitray offset anyway because of "dark current") but has low output impedance (can provide much more current).