Hi Damien,
You could try to handover a file in the payload by pltype "bin" and using the root tag <bfa:io>.
<callxmlns="">
<queryid="pdf"value="pdf"/>
<connect>
...
</connect>
<dtd/>
<pltype>bin</pltype>
<payload>
<bfa:ioxmlns:bfa="urn:com.sap.b1i.bizprocessor:bizatoms"pltype="bin">
...
</bfa:io>
</payload>
<htta>
<parid="httpheader.Content-Type"value="application/bin"/>
<parid="htta.returnpltypeforce"value="application/bin"/>
<parid="htta.returnpltypedefault"value="application/bin"/>
</htta>
</call>
In my test scenario, I'm reading a local *.pdf file and hand it over via http post call towards another B1i.
The receiver stores the handed over pdf file on it's loacal file system.
Best regards
Bastian