I am trying to convert doc to pdf/a using jodconverter and libreoffice. Default configuration allows to generate only 1.4 or 1.5 format. The only official option I have found is to use a proprietary "Document transformation engine". I don't like it because you need a windows server and ms office.
LibreOffice supports generation PDF/A via "SelectPdfVersion" property in a user configuration file (registrymodifications.xcu).