Wikipedia says firm tofu is chopped and stir-fried in Pad Thai. It is usually cooked alongside rice noodles, egg, and aromatics, allowing the tofu to absorb the dish’s sweet-sour-salty seasoning while retaining some texture.
Firm tofu is suited to wok cooking because it contains less water than silken tofu and holds its shape when cut and tossed at high heat. It can be fried first for a firmer surface, but the defining point here is the tofu’s firm texture, not whether it has been fried.
Pad Thai’s ingredient list varies by cook and region. Shrimp, dried shrimp, fish sauce, tamarind, palm sugar, bean sprouts, peanuts, and lime are common, while vegetarian versions may omit seafood and use soy sauce. Tofu skin and silken tofu are real soy products, but they are not the canonical answer to this question.