Force PDFs to always download rather than open in brower

web.config

<system.webServer>

  <!-- other system.webServer stuff -->

  <rewrite>
    <outboundRules>
      <rule name="Forcing Download for PDFs" preCondition="IsPDF">
        <match serverVariable="RESPONSE_Content-Disposition" pattern=".*" />
        <conditions>
          <add input="{REQUEST_FILENAME}" pattern="(.*)\\([^/]+)\.pdf$" />
        </conditions>
        <action type="Rewrite" value="attachment; filename={C:2}.pdf" />
      </rule>
      <preConditions>
        <preCondition name="IsPDF">
          <add input="{REQUEST_FILENAME}" pattern="\.pdf$" />
        </preCondition>
      </preConditions>
    </outboundRules>
  </rewrite>

  <!-- other system.webServer stuff -->
    
</system.webServer>