This happens in print preview and also the final document. In print preview, clicking with the magnifying glass on the red area temporarily lets you see your data, but using the previous and next buttons makes red filled areas appear on every page from then on as you navigate to it. The biggest issue is that the final document unfortunately doesn't work in the same way - zooming in on the red square just gives you a bigger red square!
(FYI I'm using VB script in a macro within Qlikview to produce the report PDF)
Any ideas gratefully received.