This option is available only for Asp.Net resize
With this behavior you can create the thumbnails of the pdf pages your user upload to the server.To use this behavior you must install the free extension "Asp Upload Pdf Ghost",
you can download this extension for free in your reserved area.
Create Thumb for Pdf:
If checked the extension create the thumbnails for pdf, but only if you setup the resize option
The format of the thumbnail image, between jpeg, png, bmp
You can setup if create the thumb only for the first page or for all the pages