​After you download and extract a pre-built distribution library from GitHub, you find two folders: build and web. PDF.js library files can be found in build folder where as web folder contains a viewer project that incudes viewer.html, viewer.css, viewer.js and related files. If you check viewer.html code, you can see that it references pdf.js library from build folder and other related css and js files as well. Visit ​https://products.documentprocessing.com/viewer/javascript/pdf.js/