I've had a chance to look at this on a system running 11.1 now (I use 11.0 on my main system) and I see the same problem in Okular (KDE4) and KPDF (KDE3), interesting that it doesn't seem to affect previews in Konqueror or Dolphin.
Apart from installing Adobe reader as some have recommended, one other app seems to open the pdfs okay and that's good old KGhostView, which is part of the kdegraphics3-postscript package. I won't try to kid you that its font rendering beautiful, but it does the job, just about.
N.B. I suspect any viewer based on the ghostscript backend will do as well.
Edit: continuing the weirdness, while playing about with Okular, I notice that the Print Preview function shows the full text.
