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I don't think that is allowed by the ePub spec. Even if a given reader is able of rendering a PDF, you'd be linking to a document that's not in the spine (because a PDF is not an allowed type in the spine) and then the reader wouldn't know what to do with the page back or forward actions, or what "page number" to show in case it has that feature.
It might be possible through fallbacks and that, but I doubt that you'd get much farther.
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