To use this option you must have GostScript and ImageMagic extension on your PHP web server
With this behavior you can create the thumbnails of the pdf pages your user upload to the server.
To verify if on your server you have the required extension you can use this page: Check ImageMagic and GhostScript InstalledTo see how install the GhostScript and ImageMagik extension see this page: Install ImageMagic and GhostScript
Create Thumb for Pdf:
If checked the extension create the thumbnails for pdf, but only if you setup the resize option
Thumb format:
The format of the thumbnail image, between jpeg, png, bmp
Pdf pages:
You can setup if create the thumb only for the first page or for all the pages
