Hello,
As is explained on the documentation these methods: addRTF, addDocx, addMht and a few others can't be used to convert the document to PDF. Because they use tha altchunk tag that is not compatible with the conversion plugin. You can use embedHtml and mergeDocx to add HTML or an existing DOCX to a DOCX.
If you want to use the altchunk tags and transform to PDF you'll need to use the method transformDocxUsingMSWord on a Windows server.
Regards.