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)