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.