Arjit,
If you are using Ariba ASN Adapter to get cXML from Ariba then you have two options within Adapter to handle Attachment:
1. Download to a Folder
2. Embedded in cXML (Base64)
I would suggest to use 1st option wherein you can download the attachment directly from Ariba.
Mount the Documentum Folder on your PI System (Basis/Network team can help) and Provide its Path in Adapter. This way ASN Adpter will download attachment directly on Mounted location which is nothing but your documnetum.
Avoid Base64 as it will increase your payload size.
Thanks,
Sunil Singh