Thumbnail not generated for pdf files

hi
I have uploaded my first pdf file (new downloadable product) – a little ebook which has a proper cover.

Of course, I was expecting that the cover of my ebook will be used by WP to generate a thumbnail for the file.

However, nothing has been generated and the file has no thumbnail image.
I spent an hour searching online – and there is heaps of posts about this issue. Some say that there is way to force a thumbnail to be generated – except that I can’t find any such settings. Other comments simply complain that WP no longer generates thumbnails.

I don’t have any thumbnail problems with jpg / png, or even mp3 files – it looks like it’s only pdf that doesn’t get a thumbnail.

Could someone clarify this for me?
thanks

The page I need help with: [log in to see the link]