pdfsplit fails to start
That file only has binaries for PPC and 32-bit Intel, so it won't run on Catalina. 

You need to see the following line for a 64-bit binary:

Mach-O 64-bit executable x86_64


Either see if there's an updated version from the developer, or use one of the many other command line tools for splitting PDFs: GhostScript, CPDF, or even python tools that call MacOS Quartz routines. 

Off the CLI, there's even an Automator action that will do it.

Here's my own python script, which runs natively on Catalina, creating a folder with all the split pages inside.

https://github.com/benwiggy/PDFsuite/blob/master/Automator_Scripts/splitPDF.py