Are you having trouble generating thumbnails for your PDF files in the WordPress media library? I found this great plugin that allows you to regenerate them all at once or also do them individually directly in the list view of the media library.
Personally, I prefer to regenerate the thumbnails individually as show below, but that’s simply because some of the PDFs I’m working with already have thumbnails.
Very easy to use and a nice freebie too! So no more awful broken image style icons in your media library. You can even use these on the front end of your site when you embed links to those PDFs.