Both Cdintf and PDFCreactiveX should be marked as "Apartment Threaded" by adding the following entries in the registry:
For Cdintf
HKEY_CLASSES_ROOT\CDIntf.CDintf Key Name: ThreadingModel Key Value: Appartment (string)
HKEY_CLASSES_ROOT\CDIntf.Document Key Name: ThreadingModel Key Value: Appartment (string)
For CdintfEx (Version 2.1 of the Converter)
HKEY_CLASSES_ROOT\CDIntfEx.CDintfEx Key Name: ThreadingModel Key Value: Appartment (string)
HKEY_CLASSES_ROOT\CDIntfEx.Document Key Name: ThreadingModel Key Value: Appartment (string)
For PDFCreactiveX (the PDF Creator)
HKEY_CLASSES_ROOT\PDFCreactiveX.PDFCreactiveX Key Name: ThreadingModel Key Value: Appartment (string)