I did not manage to get XPC to work inside a PDE plugin.
The XPC bundle must be inside the Contents/XPCServices directory of the main application bundle, but in my case, the main application bundle varies - depends on the app that opened the PDE Plugin, e.g. TextEdit or Preview. Putting the XPC bundle inside the PDE bundle did not work.
Due to this limitation, I have redesigned the flow of operation inside the PDE plugin. It now calls an external application (standalone) to do things that require network connection.