Hello,
watermarkDocx (https://www.phpdocx.com/api-documentation/docxutilities/insert-watermark-Word-document-with-PHP) works only with DOCX documents.
There's no method in phpdocx to add a watermark to a DOC file. In this case, the best approach is transforming DOC to DOCX and then use watermarkDocx to add watermarks. You can transform DOC to DOCX using the conversion plugin (https://www.phpdocx.com/documentation/conversion-plugin/information).
watermarkRemove (https://www.phpdocx.com/api-documentation/docxutilities/removes-watermark-Word-document-with-PHP) can remove watermarks from DOCX documents, but there's no method available in phpdocx to remove watermarks from PDF files.
Regards.