Jorpho wrote:

Without knowing what steps you have taken or what results you are seeing, the best anyone here can do is randomly guess.

Here is what is in the conf file:

[printer]
printer=true
dpi=360
width=85
height=110
#cox
#printoutput=PNG
printoutput=PRINTER
multipage=false
docpath=.
timeout=1000

[parallel]
printer1=\\velocity2\NitroPDF
parallel1=file dev:lpt1
parallel2=disabled
parallel3=disabled
dongle=false

Once in DosBox, I start Windows 3.1. Going to Control Panel | Printers, I see:

However, this does not work.

I don't know why you expected that to work. This feature is definitely isn't documented very well, but then it is also completely unofficial.

A Google search for "dosbox print to pdf" (no quotes) turned up https://blog.dummzeuch.de/2014/03/03/printing-from-dosbox/ , but note that MB6 is very old and you should probably keep using SVN-Daum (though the latest version of the SVN-Daum build also has problems).

EDIT: The "official" documentation is actually included with the SVN-Daum version. Go to the DOCS subdirectory, then "SVN-specific manuals", then open index.html and look for the link to "Parallel Port passthrough, file rediredtion and Virtual Printer patch, logging enabled". (Or just keep going into the "data" subdirectory and open pppatch.html.)

To be clear, if you're not printing directly to a real printer (and since you want to use "NitroPDF", I assume that is not the immediate goal), you should be using the "virtual printer" and not any parallel port passthrough.