Check out the following relevant threads:
https://groups.google.com/d/topic/pdfnet-sdk/XH8Tw_K_LvQ/discussion
https://groups.google.com/d/topic/pdfnet-sdk/RS3VfVFhXDM/discussion
In summary you could use ‘pdftron.PDF.Print.StartPrintJob’ from a worker thread, but there are also other options (e.g. use MS XPS Print API etc).