Follow up to this post from January, there is a fix using a modified web.config in the Media folder. Simply add portion inside and including httpProtocol:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <system.webServer>
    <handlers>
      <clear />
      <add name="StaticFile" path="*" verb="*" modules="StaticFileModule,DefaultDocumentModule,DirectoryListingModule" resourceType="Either" requireAccess="Read" />
    </handlers>
    <httpProtocol>
      <customHeaders>
        <add name="Cache-Control" value="no-cache" />
      </customHeaders>
    </httpProtocol>
  </system.webServer>
</configuration>

Now Chrome will not cache the PDFs. Seems to work. Thx!