The PDF in question has a few text blocks that have a grey background and a black border and there is vertical line that should not bee shown. This occurs on page 6 and 8. It does not appear that this is related to transparency groups or knockout. My best guess is that this is cmyk issue as 0 0 0 1 k and 0 0 0 0.1 k are the only color values specified. Further investigation is needed.