Pad Thai is a very popular dish served in almost every Thai restaurant. It's my favorite "Thai" food and I have always thought it is a Thai dish.
I never know it's actually more of Chinese food, and I bet few non-asian people know about this.
If Westerners believe that pad Thai symbolizes Thai cooking, many Thais agree. "Whenever we try Thai food," says Nick Srisawat, a native of Thailand who now oversees a large Thai restaurant group in the Washington, D.C., metropolitan area, "we try pad Thai first, because that is a way to judge how good a restaurant is. That's true all over the world—except in Thailand." Because pad Thai is a specialty dish in Thailand, many restaurants choose not to compete with the street-food vendors, who make and serve only pad Thai all day long and thus have perfected the recipe.
Pad Thai is really nothing more than a regular noodle dish, one that is not even native to Thailand. Its full name, kway teow pad Thai, hints at its possible Chinese origins; kway teow, in Chinese, refers to rice noodles. It is likely that some early version of the dish came to Thailand with settlers crossing from southern China, who brought their own recipe for fried rice noodles. Certainly the cooking style—stir-frying—is Chinese, and most food historians credit the Chinese with the invention of noodles. And, as Chombhala Chareonying, former Minister-Counsellor at the Royal Thai Embassy in Washington, D.C., points out, Thai food is basically Indo-Chinese in origin. The cooked meats and vegetables in pad Thai resemble dishes prepared by the Cantonese and Tae Chiew (Chao Zhou in Mandarin) from China's eastern Guangdong province. Nevertheless, the flavors and textures are pure Thai.
Greeley (2009) The Origins behind Pad Thai :