I've done up a document in Illustrator with two art boards. One has a black background, the other a white, with opposite coloured text on each. For some reason, every time I save the file as a PDF, the white text gets changed to black when I view it in anything other than Illustrator.
The text is all separated and even on different layers, and converting it to paths doesn't seem to have any effect. Nothing else in the document, including other white objects on the same page, sees any sort of colour change.