This demo is using:
Web SDK
Package: Page ManipulationCreate a Thumbnail from a PDF Using JS Demo
A demo for creating a thumbnail image with WebViewer, a JavaScript-based PDF SDK, which uses the loadCanvas API to create a high resolution JPG or PNG image of a PDF page.
Documentation
Detailed information on using the products and features in this demo.
More Resources
Implementation Options
Server SDK can also help you create thumbnails at scale.