thank you for this feedback.
I've reproduced the problem and I've logged it under:
Bug 104599: Opening a PDF page in a new browser doesn't add the browser to Manager.Browsers collection
it does not seem to be added untill the window has finished loading the pdf file
However, it seems that it doesn't get added after any period of time which is obviously not a good thing.
I hope we'll be able to provide you with a fix for this in the form of an internal release soon enough.
All the best,
the Telerik team