Version: (using KDE 4.3.0) OS: Linux Installed from: Ubuntu Packages PDF offers the opportunity to "tag" text, which is commonly used for complex text displays like math formulas etc. Such text is "tagged" with a less visual, more textual reresentation, like TeX notation. See http://www.maths.mq.edu.au/~ross/5019-e-mmap.pdf for an example document. When searching that PDF, the term "\setminus" should find all occurences of the "\" symbol (of set theory). This doesn't work within Okular. It works with Evince. Select & Copy the "\" sort of works as expected, i.e. the text that is copied to the clipboard is "\setminus" or at least part of that.
This work fine for me in Okular 0.19.0 (KDE Applications 4.13.0) so i'm closing the bug. If you update to such version and can still reproduce the problem please reopen this bug and we'll investigate further. Thanks for caring about Okular :-)