So you're using REST to get the Webi documents? Which version of BO (including service pack!) are you using? The RESTful web services have evolved as service packs have been released.
The RESTful Webi documentation for 4.1 SP4 can be found here: SAP BusinessObjects RESTful Web Service SDK User Guide for Web Intelligence and the BI Semantic Layer. The documentation for other service packs can be found at http://help.sap.com/bobip.
Go to "Web Intelligence RESTful Web Service API Reference" and then "Refreshing Documents" to get information about how to retrieve and set the parameters and refresh the report prior to downloading it into a pdf.
-Dell
