title: RStudio: open vignettes in custom PDF viewer
body: <p>When clicking on <code>pdf</code> of an R package vignette via the <code>help</code> menu tab in RStudio, the vignette always opens in the built in PDF viewer of my system (<code>Preview</code> on macOS). However the default PDF viewer of my PC is set to a custom application. </p>
  
  <p><code>Sys.getenv()</code> returns the following</p>
  
  <pre><code>R_PDFVIEWER                     /usr/bin/open
  </code></pre>
  
  <p>When calling <code>/usr/bin/open</code> manually with a random PDF file from the command line, the custom PDF viewer (which is set as the default one on my system)  is used. </p>
  
  <ol>
  <li>Why does RStudio use the built-in PDF viewer and not the system-default?</li>
  <li>What do I need to change so that RStudio uses the system-default PDF viewer? </li>
  </ol>