I haven't found an example HTTP URL where it works, it's either an assertion failure or the document silently doesn't load. The assertion failure cases might work in a release build, but it seems to trigger a "should not happen" case in the code, anyway.

The auto-test tst_QPdfDocument::loadAsync() opens a local file through QNetworkAccessManager, which does not seem to be sufficiently asynchronous to cause any trouble. That test also fails if using an actual remote URL.