I am not sure what might be causing this issue. How do you define the pdf source in the IFrame element - is it with an absolute or relative URI? If the URI isn't absolute, can you try defining the absolute path to the PDF document and see if the issue is resolved? Also, you might be experiencing cross-domain issues so you should check your cross-domain policy as well.
Unfortunately, since I can't reproduce the issue on our side I am merely shooting in the dark. But if you manage to create a sample reproducing your scenario and the issue, we will be able to further investigate it.
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