After hitting upon the right search terms, we needed to install the OpenOffice lib and unoconv.
https://docs.moodle.org/32/en/Universal_Office_Converter_(unoconv)
Hope this helps!
Searching for Unoconv will provide you with lots of info, I just didn't find it in my initial searches.
There are a few things your site admin (or server admin) should keep in mind, especially with properly setting up unoconv. There's a lot of discussion about this in the Assignment forum of this community. It can be a bit tricky managing the unoconv processing queue, event listeners and the like. There should be a lot of documentation on this in the Moodle Docs: https://docs.moodle.org/33/en/Universal_Office_Converter_(unoconv)
We had this issue too. Somehow the document converter plugin was turned off during an upgrade so it wasn't even operating. We also had updated the server and something went wrong with headless libre office and unoconv. We had to fix both issues.
Site Admin > Plugins > Document Converters. Make sure everything is enabled just in case it was accidently turned off during upgrade.
Also, when testing be sure to make slight changes to the assignment submission each time. If it fails once and produces a blank PDF it will continue to display the blank pdf until it notices something new and tries to re-process the doc or text to generate a PDF.
