<rewrite>
  <rules>
    <rule name="PDF" stopProcessing="true">
      <match url="PDF/(.*?)/(d*)/(.*?.pdf)$" />
      <action type="Rewrite" url="SISPdf.aspx?id={R:2}&fn={R:1}" logRewrittenUrl="true" />
    </rule>
  </rules>
</rewrite>

http://myserver:8089/PDF/ABC/123/DEF_456.pdf

http://myserver:8089/SISPdf.aspx?id=123&fn=ABC

<match url="PDF/(.*?)/(d*)/(.*?.pdf)$" />

<match url="PDF/(.*)/(d*)/(.*).pdf$" />