All i understand is you will send PDF to pi to convert and get xml. This is clear.
How the whole execution starts? from where are you reading the pdf in ECC? are you reading the pdf from app server? where the pdf is stored?
can you tell me what is the next step, once xml is received in ECC?
for example, if you are going to store the response xml in table, you can do it using async interface. so you can avoid response. Please note, if 10 pdfs files are there in ECC, write the proxy code to call interface 10 times in this case.