Hello,
The conversion from HTML to DOCX is done through internal PHP classes without using LibreOffice/OpenOffice.
UPDATE: phpdocx 9 added a new conversion plugin to transform DOCX to PDF using native PHP classes.
phpdocx only needs LibreOffice to use the conversion plugin based on LibreOffice to transform DOCX to PDF, DOC to DOCX, DOCX to ODT,...
Regards.