Fountain files are quiet often generated as backup files from screenwriting apps / online tools and thus are really not that uncommon, although FDX format is probably the favorite exchange format for screenswriting.
If you need to export these files to PDF for printing, you just need to load them up in one of the screenwritting tools and just use the PDF export function or PDF printer. Most modern tools should be able to read .fountain files just fine and thus capable of fountain to pdf conversion.
Updated: August 17, 2023