When I found a couple of years ago that WordPress will create thumbnails of uploaded PDFs, I was so happy, thinking I wouldn't have to make and upload separate PDF cover images to have a thumbnail display on my site as the link to a PDF. After some struggle I found that the images created from the uploaded PDFs aren't accessible for display on any public webpage. It seems logical to me that since these files are created when I upload a PDF, I should be able to access them, but this isn't possible. I want to clarify that I don't what to embed the PDF (some are very large), I just want the automatically-generated thumbnail to the be the link to the PDF.
I tried the plugin PDF Thumbnails, but it's clunky at best and it doesn't use the automatically-generated images; it has to create its own.
This seems like such a simple and obvious thing to do so I'm really surprised that this isn't possible "out-of-the-box". If I see the thumbnail in admin, why can't I see it on the live site, too?
The post Force documents to appear in Featured Image dialogue doesn't help me because the Twenty Twenty-three theme doesn't have the typical functions.php file and post-type template files, at least that I can find. I should also note that I am using multisite.