I am not sure I properly understand you comment.
The snippet you pasted seems okay to me. Please note that with this setup the Loaded event will be raised and the stream would be disposed if you open a new document, for example. Like Kammen said, documents are loaded asynchronously in RadPdfViewer so the fact that part of the document is shown does not mean it has been loaded. In fact, the Loaded event is only raised when the stream used by DocumentSource is no longer needed.
I hope this makes things clear.
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