Since [1] PDF.js does not dispatch events to the DOM any more. Therefore
the client needs to listen for the `documentloaded` event from
`PDFApplicationViewer.eventBus`. The `eventBus` property is only
available once the viewer is initialized, so the client needs to wait
for `initializedPromise` to resolve before checking it.

[1] https://github.com/mozilla/pdf.js/pull/11655