Pit's f-air shield stun = 7 frames (according to
this)
Pit's L-canceled f-air landing lag = 8 frames (according to
this)
advantage on block = -1 frames
So, the opponent has one frame of advantage on you if he blocks your f-air. This assumes that you do Pit's f-air as close to the ground as possible, and it also assumes that Pit's f-air is not stale and would do 13 damage (otherwise you might inflict 1-2 fewer frames of shield stun). And for my last assumption/guess, I'm assuming that no grab in PM is faster than 7 frames. So theoretically, to trade with your opponent's 7 frame grab, you would need a 7+(-1) = 6 frame option. But... grabs outprioritize other attacks on the same frame (the super-armor effect on grabs), so you actually need a 5 frame option.
Pit's ground options that are 5 frames or faster (according to
this and
this):
1 frame: begin to dash away or dash through your opponent
1 frame: up-b (invincible 1-6, hits frame 5)
2 frames: spot dodge (invincible 2-15)
3 frames: usmash
4 frames: dsmash
4 frames: roll away or roll through your opponent (invincible 4-19)
5 frames: jab
5 frames: fsmash
Apparently Pit's jump has 5 frames of startup (according to
this), so any of Pit's aerial options are likely too slow to outspeed a shield grab. Wavedashing might work, but I think dashing away is just better. Crouch is another option for some characters, but I don't think Pit crouches low enough to duck any grabs.
Personally, I use dash away and jab the most. I like using dtilt too (6 frames), since it combos to almost anything, but that relies on your opponent not being frame-perfect on their shield grab.
Now, all of this is assuming your opponent goes for a shield grab. There are plenty of other options out there faster than frame 7, like continuing to hold shield, spot dodging, rolling, wavedashing OoS, Samus/G&W/Bowser/Pit/etc up-b OoS, some up-smashes OoS, etc. But now you know how to beat a shield grab!