PAC

PAC (sometimes called BIN or PAK) is an archive format used in the PS2 versions of Persona 3 and Persona 4. In most cases, 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.

RMDEditor
RMDEditor.exe is a program created by OfTheSeventh and maintained by ThatTrueStruggle. It features a GUI similar to BrawlBox and is capable of opening PAC|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.

Amicitia
Featured article: Amicitia (program) Amicitia is TGE's all-in-one editor for Persona formats. Currently, it supports almost everything that RMDEditor can, but without having to deal with nodes. It is able to open PAC files to extract/edit the TMX, SPR, RMD or ACX files inside. You can get it here.