I put a example file at http://www.stanford.edu/~tdillig/example.pdf. In evince and acroread, this file shows a large blue semi-transparent rectangle. In okular, this file appears empty. I was told by the Okular team that is is an underlying bug in poppler. I am using KDE 4.2.2 okular.
Splash backend bug
Fixed in poppler >= 0.11.3
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