OK, issue solved:

my etc/hosts file had my domain listed as a loopback.

I had this line in hosts in order to get mail generated locally for another application:

127.0.0.1 localhost mydomain.com

This resulted in WP trying to use loopback instead of the real IP to acquire the RSS content for the pdf. That’s why the pdf ended up blank.

All sorted now; thanks to Mike for pointing the way.

Paul