"PE" in this context stands for "Portable Executable". (For those not in the know, such as myself.)
The way a PE file is defined is interesting. They managed to cram a completely different file format into the DOS 'MZ' format. Including the NE and LE formats. I think the NE one had a bunch of sub targets (OS/2, win32s, etc).
https://wiki.osdev.org/PE https://wiki.osdev.org/NE
It is quite the nesting doll of executable formats. If I remember correctly PE also holds .NET in some way as another sub format.
(sigh at how acronyms are the source for endless insider snark.)