sorry for the trouble.
Unfortunately the settings for the default application and the default application for printing aren't always the same in Windows 10 / Windows 8.
PDFCreator just looks up the default command for printing file type in the registry and executes the command it finds there.
First it will look at the default/standard value inside Computer\HKEY_CLASSES_ROOT.docx which for me is Word.Dokument.12
If the value is different for you e.g. LibreOfficeDocument then you will need to replace that part of the next path accordingly:
The it will look up the command in e.g. Computer\HKEY_CLASSES_ROOT\Word.Document.12\shell\Printto\command
if there is no "Printto" entry it will check for Computer\HKEY_CLASSES_ROOT\Word.Document.12\shell\Print\command which sends the document to the default printer, but since PDFCreator temporarily sets itself as as default printer if there is only a "print" command, there just needs to be a valid command in either of the two paths.
For me this points to MS Word but I have also used LibreOffice successfully in the past, if you need the printing command for LibreOffice and can't find it in the online documentation let us know and we will help.
What happens if you right-click onto a docx file and select "print" instead of "convert with PDFCreator", does this also use WordPad or does it use LibreOffice?
As a work around you could open the documents in LibreOffice and print them to the PDFCreator printer from there.