I’ve seen a few cases where reports exported or rendered to PDF have distorted charts and images. We’ve played with fonts and chart properties have sort of accidentally corrected the issue but without really getting to the core problem. There’ been quite a bit of traffic generated on the MSDN forums in the past and the answer is in there if you look hard enough. A friend and industry associate, Dave DuVarney, researched the problem and figured it out. I’ve seen cases that were worse than this one. Here are before and after screen shots of a chart report rendered as a PDF:
Notice that the fonts on the X axis are dithered. As I said, this example isn’t extreme.
Now the corrected version:
Turns out that changing the background color of a chart changes the image scaling and corrects the problem. Follow his blog and the details here.