Hello,
It seems the text you are adding has that weird characters and for any reason we don't know the PDF reader included in Firefox show them, but not other PDF readers.
We recommend you to check the DOCX file with a HEX editor to find that characters to be able to clean them. Maybe if you also send the PDF to the Mozilla team they can guess why the inline PDF reader included in Firefox is showing that characters but other PDF readers not.
Regards.