A man is reading a book while sitting on a bench outside of a bookstore.
A man is reading on a bench outside of a store.
Reading a book while sitting on a bench outside of a store can only mean you recently purchased something from that store thus implying that the store is a bookstore.
A bookstore is a type of store, so a person outside of a bookstore can also be said to be outside of a store.
A man is reading a book while sitting on a bench outside of a bookstore is same as man is reading on a bench outside of a store.