"The Product Backlog lists all features, functions, requirements, enhancements, and fixes that constitute the changes to be made to the product in future releases"
This list can be (and I believe in practice very often are) translated into User Stories, but they don't have to be, right? in other words the Product Backlog does not require to have User Stories, is this correct?
Yes, it is correct. Most of the time, you will have PBI as User-story, but it could be "bug" ; "spike" ; performance or security enhancement... You can very often turn the PBI as a user-story, but it is not useful all the time.