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KPDF is part of the kdegraphics package. It should be packaged by the distribution you use, so you should almost never have to download it and compile from the source code.
In case you want to get the lastest stable source code you can grab the kdegraphics package at ftp://ftp.kde.org/pub/kde/stable/latest/src
If you want to get the code from the KDE subversion repository, use /branches/KDE/3.5/kdegraphics/kpdf/
If you want to develop new features you should have a look at okular pages, as that is the program that will succeed KPDF.
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KPDF - more than a reader