Found this list of CAD related software. There’s a list of viewers part way down the page. I’ve used QCAD before. It’s an open-source program similar to AutoCAD. It’s website claims it can read and write DWG files, so it can probably convert them to several other formats too.
Edit: Yup, if you look on this page, it lists some command-line utilities that are included with QCAD, including one called dwg2pdf for bulk converting of DWG files to PDF.