The PBGC is a government agency that guarantees payments promised in defined benefit pension plans. Information about the agency is available in the PBGC web site.
The PBGC's Legal Filings and Decisions page provides briefs, pleadings and motions filed by the PBCG since 2011, as well as selected judicial decisions issued in response to PBGC filings. Otherwise, you can get them from the various courts and online databases providing case filings and judicial opinions. Lexis has a database of Pension/Benefits-related cases including PBGC decisions (PENBEN;CASES).
The PBGC also posts "Significant Guidance Documents" issued since 1981.
Regulations: The PBGC posts proposed and final regulation published in the Federal Register since July 1, 1996 and the relevant sections of the Code of Federal Regulations. You can find older regulations by searching back issue of the Federal Register or historical editions of Title 29 of the Code of Federal Regulations (see "Federal Register" and/or "Code of Federal Regulations").
Appeals Board Decisions: The PBGC posts a database of Appeals Board Decisions decided by all three board members (but not single member decisions) since October 1, 2002.
Opinion Letters: The PBGC provides a comprehensive database of Opinion Letters. They are also searchable on Lexis (LABOR;PBGC) and Westlaw (FPEN-PBGC) from 1994 to the present, as well as the BNA Benefits Practice Center (subscription only). Once upon a time I used to get PBGC Opinion Letters from the slim volumes at the end of the CCH Labor Law Reporter titled, "ERISA Update: PBGC Advisory Opinion Letters." You can still find them there, if you have the volumes on hand.