Puerto Rico is a United States territory and all Puerto Ricans are US citizens. Without getting into all the legal details of the relationship between the US and Puerto Rico, in simple terms Puerto Rico is much like any other state of the US. The Puerto Rico currency is the US Dollar.
If you are a US citizen YOU DO NOT NEED A PASSPORT. Entering Puerto Rico is the same as going to Florida, New York or California.
If you are not a US citizen, but you have a Visa to enter the United States you do not need any other documents to enter Puerto Rico.
If you are not a US citizen and you do not have a Visa to enter the US and you need one to enter the US, then you must apply for a Visa to enter the US in order to visit Puerto Rico.