ZIP a directory on-demand?

Hi! I'm wondering if there's any way to make a directory download as a ZIP file. To be clear, I know how to ZIP a folder and make it available on the server, but I would like to know if there's a configuration or application that will make a non-ZIP'ed directory download as a ZIP file when a visitor clicks a link? Imagine a folder full of PDFs that get individually updated and replaced now and then, they need to be available to read online, but also the whole folder downloadable as a ZIP. Would be nice not to have to re-zip every update.