This was reported at http://fofou.appspot.com/sumatrapdf/topic?id=872322 Manuals from http://www.usa.canon.com/cusa/support/consumer/digital_cameras/powershot_a_series/powershot_a720_is#BrochuresAndManuals (e.g. http://gdlp01.c-wss.com/gds/2/0300003282/01/ib65-en.pdf) show a watermark (a "copy" text across the page in gray, blended with the background). I verified that Adobe Reader doesn't show the watermark on screen but it does show it in print preview which I guess means this should only show up when printing. That would mean that there needs to be a switch for app developers to tell rendering engine whether the watermark should get included or not.
This is related to Optional Content, a pdf 1.5 feature that we don't implement yet.
*** Bug 691711 has been marked as a duplicate of this bug. ***
Fixed in commit a287d03f33f05a842fdd725f1ab95dbeafa9fe5b.