Why is the viewer showing "Network Failure"?

This error likely means the initial document does not exist (we may have updated the SDK package). Try setting initialDoc to something like https://pdftron.s3.amazonaws.com/downloads/pl/demo-annotated.pdf

WebViewer({
  path: '../../../lib',
  initialDoc: 'https://pdftron.s3.amazonaws.com/downloads/pl/demo-annotated.pdf'
}, document.getElementById('viewer'))
.then(function(instance) {
  // load the PDF from URL, Blob, Filesystem or Base64 data
  instance.loadDocument(e.target.value);
});

which is a demo document online, or to another local PDF file on your system within the root directory tree for the project.