With the webkit backend, when I try to open a pdf in the embedded okular viewer, okular does appear until the page is refreshed or the window resized, only a gray page appears. The KHTML backend on the other hand behaves correctly. Reproducible: Always Ubuntu Precise Pangolin, KDE 4.8.2
Sorry, but this is a distribution specific issue. Unfortunately all debian based distributions uses a very outdated version of kwebkitpart [1]. As such, these distros never benefit from all bugs that have been fixed since that release [2]. [1] http://packages.debian.org/source/sid/webkitkde [2] https://projects.kde.org/projects/extragear/base/kwebkitpart/news