title: Setting the default PDF viewer for rstudio
body: <p>Using knitr in Rstudio, when an .Rmd file is knit to PDF, the resulting file viewer is evince.  I would very much like to change this to okular, but I cannot find a way to make this happen.</p>
  
  <p>Looking at the Sweave settings, I see "system viewer" noted for the preview, but okular is already set as my default system viewer.  I've also checked the default viewer with xdg-mime and okular is default there as well.  In all other instances, okular is the default but Rstudio/knitr always used evince.</p>
  
  <p>How can I change this behavior?</p>