You are invoking an executable whose name has a space character in it. Usually those files originate in Windows systems but Linux supports them.
That \ character there (note direction of slant) is trying to escape the space character. But in your shells (Or at least in that invocation) you didn't do it correctly. Siard did by putting the executable into his PATH, but you don't have to. Enclose the whole command line in single quotes and give it a try.
Rodolfo Medina:
> Brian:
> > ./PDFtoMusic\ Pro gives me "Segmentation fault".
>
> To me instead it says:
>
> -bash: ./PDFtoMusic Pro: No such file or directory
I can only say that it works fine here in Buster.
>From ~ or any other directory I run this command:
$ PDFtoMusic\ Pro
These are all its files:
~/.config/ACAMPREF/*
~/Myriad Documents/PDFtoMusic/*
/usr/bin/PDFtoMusic Pro
/usr/share/PDFtoMusic Pro/*
/usr/share/applications/pdftomusicpro.desktop
/usr/share/fonts/truetype/myriad-online/*