Hello,
You need to set rw access to the .config folder of the home folder of the user that is running the web server. This path is set in your server config; the most common in Windows using apache is httpdocs.
We also recommend you to try setting the method option as script and enable the debug mode to get more info in the logs of Apache:
http://www.phpdocx.com/api-documentation/format-conversion/convert-Word-docx-into-pdf-with-PHP
What version of LibreOffice are you using?
Regards.