Hello,
If when you open the PDF it returns an error (such as 'Unable to open document') then it's being generated corrupted. Please follow the steps explained on:
http://www.phpdocx.com/documentation/conversion-plugin/debugging-libreoffice
the debug it. If you prefer that we check it, please send the DOCX you're trying to transform to contact[at]phpdocx.com
Regards.