Command Line Tool to Batch Convert .EML/.EMLX/.MBOX to Searchable PDFs?

I need to convert about 500k emails into searchable PDFs. By ‘searchable’ I mean that macOS will be able to scan them for specific words rather than simply treating them as an image. My searches, thus far, for a tool to do this have ended in proprietary database apps and over-priced sketchball x-to-pdf converters which basically perform the built-in macOS functionality of Print To PDF. Is there a single tool or two complementary tools that could be used together in Terminal to just batch convert all the emails to searchable PDFs?