Place noodles in a large heatproof bowl. Cover with boiling water. Stand for 5 minutes, or until just tender. Drain. Rinse under cold water. Drain well. Set aside.
Combine the fish sauce, sugar and lemon juice in a jug. Set aside.
Meanwhile, toss chicken with half the oil. Heat a large wok over a high heat. Add one-third of the chicken. Stir-fry 2 minutes, or until just cooked. Remove to a clean bowl. Repeat in two batches with remaining chicken.
Reduce heat to medium. Add onions and remaining oil. Stir-fry for 3 minutes, or until soft. Add 2 teaspoons water, cover and cook 2 minutes. Remove the lid, stir in the pad Thai paste. Stir-fry for 30 seconds. Add carrot and snow peas. Stir-fry for 3 minutes, or until carrot is tender.
Push to one side of the wok. Add the eggs and allow to cook until slightly set. Use a wooden spoon to stir the egg until scrambled. Increase heat to high, add the noodles, chicken and fish sauce mixture. Stir-fry 1 minute until hot. Remove from heat.
Sprinkle with peanuts. Serve with bean sprouts, chilli, coriander and lemon wedges.