KPDF is part of the kdegraphics package of KDE 3.
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/3.5.10/src
If you want to get the code from the KDE subversion repository, use /branches/KDE/3.5/kdegraphics/kpdf/
KPDF is not developed anymore, if you want to develop new features you should have a look at Okular, that is the succesor of KPDF in KDE 4.
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