Pfizer Inc. said that it received two subpoenas from the U.S. Attorney’s office in Massachusetts related to charities that help Medicare patients afford co-payments for drugs, the latest company to disclose involvement in the probe. The New York-based drugmaker said in a securities filing Thursday that it received subpoenas on December 2015 and on July 2016 requesting documents related to the Patient Access Network Foundation and other organizations that provide financial assistance to Medicare patients.