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)
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 any type of BIN/PAC files to extract/edit the TMX, SPR, RMD or ACX files inside. You can get it here.
PAKPack is a commandline program that is part of the AtlusFileSystemLibrary, and can be used to edit PAK files. Useful if you want to edit PAK files in batch. You can download the latest version of PAKPack, other archive tools, and find the source code here.