The rmagick gem is a wrapper around the
ImageMagick software suite.
This gem can be used to create a thumbnail image of a PDF using the
following snippet of code.
require 'rmagick'
pdf = Magick::ImageList.new('document.pdf')
first_page = pdf.first
scaled_page = first_page.scale(300, 450)
scaled_page.write('document-thumbnail.jpg')
The scale can be adjust as necessary to the use case.