My paternal grandfather reported he naturalized in Pennsylvania. How do I find that information?

Question

Christopher (Christopf) Popp reported in the 1900 Federal Census that he naturalized to US in PA. He stated he had been in the US for 11 years. His DOB is 22 Feb 1856. He stated he was from Biederbok (Germany?)

A1 Reference Branch · Answer

NIS2236 
 Thank you for posting on History Hub! 
 Naturalization records dated prior to October 1991 from the federal courts are at the National Archives. If you were naturalized in a federal court before October 1991, you will need to contact the NARA field site that maintains federal court records for the state where you were naturalized. To determine which facility you need, please visit our webpage at https://www.archives.gov/research/court-records . Please note that no central index to naturalizations exists. Since you mentioned Pennsylvania, we recommend that you reach out to the National Archives at Philadelphia via email at philadelphia.archives@nara.gov . 
 Records for naturalizations that occurred after October 1991 are held by the United States Citizenship and Immigration Services (USCIS, successor agency to the Immigration and Naturalization Service or INS). If your naturalization occurred in October 1991 or after, you will need to contact USCIS directly. For information on requesting copies of records from USCIS, specifically those relating to your own naturalization, you will need to file a Freedom of Information Act (FOIA) request with USCIS. Please go to their webpage at: https://www.uscis.gov/records/request-records-through-the-freedom-of-information-act-or-privacy-act . 
 We hope this information assists you with your research! 
 Sincerely, Archives 1 Reference Branch [RR1R-24-41650-TJA]