I'm the author of Ubuntu Pocket Guide and Reference, which is offered as a free download at www.ubuntupocketguide.com.
When viewed using a Poppler-backed PDF viewer (i.e. Evince), some of the images in the book's PDF (v1-1) appear to be broken up by fine horizontal white lines. See for example page #146 (PDF page numbering) of the book. I've attached a screenshot from Ubuntu 8.10.
According to Help -> About, the screenshot is taken using Poppler 0.8.7.
The PDF displays perfectly in all other PDF readers, whether Free Software or proprietary.
The book was outputted using Acrobat 9, and the image quality set to JPEG Low. If you'd like to know any specifics about the outputting process then let me know.
If this is something that I'm doing wrong then please let me know and I'll fix this issue on the next version.
Cairo backend problem, works with the splash backend
*** This bug has been marked as a duplicate of bug 10942 ***