[dfe]

Correction. Glancing at some of the actual code, and also looking at the README.md seems to indicate this is in fact the IBM DOS 4.

For example, the src/BOOT/MSBOOT.ASM file has a revision history including 3.31 and has code for the 32-bit sector count.

But more to the point the src/CMD/IFSFUNC folder contains the code for the IFSFUNC.EXE which only exists on IBM DOS 4.

So it would seem that the pdf folder has absolutely nothing to do with the source code, it's just an archeological bonus.