This is a simple viewer for latex-beamer presentations that are built with the show notes on second screen option of latex-beamer.
It will take your PDF file, split it in a left and right half and render the two halves individually to the screens.
There is a manpage included that explains all the options and keybindings. Most important ones are probably:
Command line:
dspdfviewer [options] pdf-file
where the most useful option is probably -f, because it enables you to use the software with a standard pdf (i.e. not specifically built for latex-beamer).
In-Program-Controls:
Left/Right, Mouse Buttons or Mouse Wheel: Back/Forward
S or F12: Swap screens (if you see the wall clock on the projector)
B or . (period): blank/unblank audience screen
Q/Esc: Quit
Please read the installation section on the project's website for detailed instructions, broken down by operating system.
For generic from-source installation instructions, you can also consult the INSTALL file.