When you are carrying out an action that uses the Muhimbi PDF Conversion service and the request (from the client to the Conversion Service) is larger than 100MB (50MB for 8.3.0 and older versions) then you will see the following error in the event log:
The remote server returned an unexpected response: (400) Bad Request
By default the maximum file size is 100MB (50MB for 8.3.0 and older versions), but this value can be increased in the PDF Converter's config file. For details search the Administration Guide for Timeouts and File Size limitations.
When you write your own client side code then you will need to make sure your Web Services framework of choice is configured to accept potentially large responses.