Two kinds of ebuilds are available for dspdfviewer: the git ebuild (with version number 9999), which downloads and installs the latest git version of dspdfviewer, and regular ebuilds with normal version numbers.
To install, move the ebuilds in the “gentoo” subdirectory into an overlay (for more information on overlays, see the gentoo manual) in the “app-text/dspdfviewer” directory. Then, unmask the version you want and run “emerge dspdfviewer”.
The inclusion request for the main Gentoo repository is filed under bug 520120 in the Gentoo bugzilla.