What if someone doesn't have papers (ssn, green card, visa, etc) can they go to Puerto Rico with their ID/Drivers license and enter the country with no problem?

According to at least one Airlines webpage... a Drivers license should qualify.

United Airlines - U.S. Territory Travel Document Requirements (http://www.united.com/web/en-US/content/travel/airport/id/travel.aspx)

Correct, that's what I read, but I was also told you need to go through customs/immigration when you return, so that's why I'm asking...
According to at least one Airlines webpage... a Drivers license should qualify.

United Airlines - U.S. Territory Travel Document Requirements (http://www.united.com/web/en-US/content/travel/airport/id/travel.aspx)

I can't say if Customs is invovled... as I've never flown to Puerto Rico before, I always deal with them because I go to Europe annually... But you present your ID. The difference is which line you get into.

You are entering a foreign country. A current passport would be helpful. Yes, when you return you will have to go through US customs, best to have your US passport handy.

US or Puerto Rican citizens can travel either way with just a drivers license or other pictured ID. Guam, US Virgin Islands and any other protectorate-Midway, Samoa and others is the same. Canadian citizens need a passport.