This sample application uses a PDF merge service hosted on Google App Engine to merge two or more PDF documents stored in Force.com. This application was demoed during the 'Cloud to Cloud Integrations with Force.com' webinar (http://wiki.developerforce.com/index.php/Webinar:_Cloud_to_Cloud_Integrations_with_Force.com) and additional details about it can be found here - http://blog.sforce.com/sforce/2011/06/cloud-based-pdf-merge-solution-for-forcecom-.html.