Using PDF as the featured image for post thumbnail

I would like to figure out how to use a pdf as the featured image in a post thumbnail featured image. Is there a way to add support for this in functions.php or somewhere in the wordpress core?

I’m creating a free resource for teachers and they will be uploading pdf’s to share with other teachers and I would like to use the featured image thumbnail in the post as a preview for the download before a teacher decides to download the file.

I’ve already added a lightbox to the thumbnail to make it clickable into a lightbox to view it better, but I can’t figure out how to allow pdf’s as featured image.

Help would be greatly appreciated. Thanks in advance.