Hi,
Thanks for your patience.
I have again tested the scenario using latest release of Aspose.Pdf for Reporting Services 2.7.0 over Windows Server 2008 R2 where I have configured SQL Server 2008 R2 Reporting Services and as per my observations, the PDF export via Aspose.Pdf option is properly appearing and I can also export one of my sample .RDL files to PDF format without any issue. For your reference, I have also attached the images showing my working environment.
I would suggest you to please try using the latest release of Aspose.Pdf for Reporting Services 2.7.0 amd in case you still encounter any issue, please feel free to contact.