Elco wrote: Thanks for the comments, and the solution.
In the Linux Mint menu, the LinuxCNC documents are called like:
/usr/bin/see /usr/share/doc/linuxcnc/LinuxCNC_integrator.pdf
But in Linux Mint, 'see' does not follow the default setting for *.pdf
and opens gimp ('see' has its own defaults). So setting the default did not help.
Replacing this with
/usr/bin/xreader /usr/share/doc/linuxcnc/LinuxCNC_integrator.pdf
solved the problem.
That looks like the PDF is being opened with a program called
see (/usr/bin/see) rather than allowing it to be opened with the default application. I don't know that command. I wonder if man see or see --help or see /h will give more info? Sounds like there is a config file somewhere to point a file type to the right application