The "hello world" example is a minimalistic application of the PDF.js project.
The file helloworld.pdf originates from the GNUpdf project and contains a
simple and human-readable PDF.
Instead of simply opening index.html in a browser, you must serve the page
using a web server. This can be done on your local machine without an internet
connection. In the root directory of PDF.js, run gulp server in a
terminal. The example can then be viewed using the following URL:
http://localhost:8888/examples/helloworld/index.html
Take a look at hello.js to see how to make basic calls to PDF.js.