I live in Southeast Asia and have been to Thailand many times. Pad Thai is well known to every Thai person but it is not seen all that often on their plates (or in their plastic bags, where takeaway food is conveyed). And indeed a lot of Thai restaurant menus do not list Pad Thai, or if they do it's far down the list, like "chicken fingers" in an American restaurant.

In tourist hotspots (which abound in Thailand) you'll tend to see picture menus and Pad Thai will be on them. But there are much, much better dishes to be had outside these areas. One issue is they don't all have consistent names, e.g. when I asked for a recommendation and was given an absolutely amazing dry curry crab noodle dish with egg, I asked for its name in Thai and translated it was "curry crab" which would never get you a similar dish in another restaurant. Even the venerable Pad Kee Mao, which I think is always a noodle dish in America, is sometimes a totally different rice dish in Thailand.