For the communication between the AS ABAP and the ADS running on AS Java the service user ADS_AGENT is required.
You need to specify this service user in the authentication parameters of the Destination service in the AS Java that hosts the Adobe Document Services (ADS).
You must specify this client in the Destination service.
You must specify this password in the Destination service.
If AS ABAP and AS Java (with ADS) are installed on the same server (double-stack installation or AS ABAP with Java hub), assign the role SAP_BC_FP_ICF.
If AS ABAP and AS Java (with ADS) are installed on different servers, assign the role SAP_BC_FPADS_ICF.
This distinction is for performance reasons. You may copy the required role first.
More information: Changing Standard Roles in the SAP Library
More information: Creating or Changing the Destination Service