Setting the PDF viewer
----------------------

If the environment variable DSV_VIEWER is set, dsv will launch the
program specified there.

If DSV_VIEWER is not set but DSV_PDFVIEWER is set, dsv will launch
the program specified in DSV_PDFVIEWER.

If neither DSV_VIEWER nor DSV_PDFVIEWER are set, dsv will run
xdg-open to launch the viewer.

xdg-open typically defaults to the "evince" viewer. The default
can be changed from the command line with

xdg-mime default APPLICATION application/pdf

where APPLICATION is one of the *.desktop files typically found
in /usr/share/applications/ or /usr/local/share/applications/

For example, to make xpdf the default xdg-open (and hence many
other applications) uses, one would run

xdg-mime default xpdf.desktop application/pdf