kuzmandi The basic layout and color of a company logo are usually enough to mentally locate the file (faster than reading a long filename).
Depends on your PDF's, but we are talking about a thumbnail perhaps 160x160 px (on average). Unless the logo happens to cover a large area on the front page, it would be a tiny colored blip. I have seen PDF thumbnails myself, and for typical text documents (which is what PDF is for), I found it useless and annoying ... It might be different for you if you do actually have large logo-like graphics, but for text-only definitely not ...
As noted in my previous post, this can only be achieved by having your own server with
exec() function available and ImageMagick installed ... It's not possible with PHP-only.