PAC (sometimes called BIN or PAK) is an archive format used in the Persona series. In the PS2 titles, they contain a number of TMX and SPR files which are loaded together (i.e. the Title Screen, Pause Menu, Results Screen, Battle HUD). However, the in-battle RMD models are contained in PAC files because they are also loaded with their respective ACX voice clips archive. In Persona 5, a variant of BIN is used again to contain bustups and various UI elements and scripts (in which case it is referred to as PAK).
RMDEditor.exe is a program created by OfTheSeventh and maintained by ThatTrueStruggle for opening the PS2 variant of PAC files. It features a GUI similar to BrawlBox and is capable of opening PAK/BIN, RMD and SPR files, as well as exporting and importing nodes from the files. You can download the latest version (v1.4.6) here.
Featured article: Amicitia (program)