Hello,
The stream mode works for document generation output (https://www.phpdocx.com/documentation/cookbook/improve-phpdocx-performance, DOCX as a stream section), but not as source when using the conversion plugin. You need to generate the DOCX file and then transform it to PDF.
If you need help to debug the conversion plugin based on LibreOffice, please check the documentation available on https://www.phpdocx.com/documentation/conversion-plugin/debugging-libreoffice. The most common issue is missing rw access to the .config folder of the web user.
Also please note that trying to download more than one file should be avoided (https://stackoverflow.com/questions/3663127/php-multiple-file-download). You should compress the files into a single package such as ZIP.
Regards.