Here's an example of using thumbnails on the Internet, for linking to individual items.
They are pictures, but not the ones which Paul is referring to
The site hosts a bunch of my Railroad Pictures, and they are accessed per subject page via thumbnail versions of the picture.
Site is located at this URL: http://www.railfan.net/railpix/scott/
From this main page, choose one of the subtopic pages - as they have the thumbnail links. Click on a thumbnail to view full size image.
As to the .PDF thumbnails, they are kind of similar, but instead you jump to a page in that document.
Most of the time, when creating a .PDF document, the default settings make a thumbnail frame active - or your reader may be setup to make a thumbnail frame by default.
Unless the document maker has setup thumbnails, there will be nothing in that pane. When there are thumbnails, they work just like Hyperlinks.
Problem is they are like less than 10% of the document's size, so jumping to a page may be more of a "Crap Shoot" than expected!
(meaning just what can be seen on the thumbnail!)