I would urge you to contact the Pennsylvania Dept. of motor vehicles with your question for specific advice on the logistics applciable to PA, but typically a car purchased out of state can be temporarily registered to you in that state (here Ohio) and then permanently registered in the state of residency (here PA) upon being brought into the state.
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In Ohio you can drive for up to thirty days with the plates from your old car placed on the new car. You should still contact PA to find out what they say. Good luck.
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