Thanks David, Kevin.
The whole idea behind the ExportWebMap is that you have map (composed of mapservice, featuresure, graphics etc.) on your web app and you want that map to be printed by exporting it out to a pdf or any other format first.
Since your web applications does not need to have mapping component, it seems like a custom arcpy.mapping based Geoprocesing service might be a viable solution.
Before I can suggest anything specific, I'd appreciate if you could answer the following questions:
1. do you plan to use map service layer(s) in the map document that you want to be exported out to pdf?, OR
2. is it that you have an existing mxd containing all layers pointing to local datasets (e.g. in a fgdb, shapefile, sde etc) that you want to open, zoom into the extent of a feature and export to a pdf?
Kevin:
3. in your report, how do you know what extent the map needs to be zoomed at? is that like your web app knows the extent always and you are planning that to be sent to the ExportWebMap task?
Please keep in mind that ExportWebMap task in PrintingTools service currently does *not* support returning multi-page pdf - it is always single page. If you need to return multi-page pdf, then you need to go with a custom solution.
A side note: arcpy.mapping.ConvertWebMapToMapDocument, as the name suggests, takes the same webmap_json that ExportWebMap task takes and returns a MapDocument objects that you can then export to any format supported using arcpy functions.