After having initialized the UIDocumentBrowserViewController
UIDocumentBrowserViewController* documentBrowserViewController = [UIDocumentBrowserViewController new];
[documentBrowserViewController setDelegate:self];
[[self window] setRootViewController:documentBrowserViewController];
the delegate method documentPicker:didPickDocumentsAtURLs: is not called as documented but the deprecated documentBrowser: didPickDocumentURLs: is.
So, what is wrong, the implementation in iOS 12 or the documentation?