Say you have a SSRS report in AX and you also want to have access to it from EP. The way to do this is to run the report silent and redirect it's output as PDF on the IIS side, then download it on the client's side and have the browser display it's Open/Save dialog so you can work with it.
Basically the implementation for this is done in several places in standard AX already:
1. first you have the downloadDocument.aspx page - the page that actually get called and to which the response will be written to by the server; the EPDocuGet class instantiated here is present in the dll Microsoft.Dynamics.Framework.BusinessConnector.Proxy
2. secondly you have the EPDocuGet class mentioned above; it is responsible for actually preparing the content of the documents you want to download. In its standard implementation there is a call to another class EPSendDocument which in turn get's the report run and stored as PDF; it can handle only a few standard reports AX ships with. However, you can tap here and add the calls for your custom reports. More on this in the next post.