use of IDOCS

Dear All

Am an Abaper but new to IDOCS or EDI. I have a requirement of interface with non SAP System. I need to receive 3 scanned documents from non SAP system to SAP System. The documents are , police report, evacuation and invoice. After receiving need to post the invoice in SAP FI as a parked document.

My question is what information can i provide or is needed to the non SAP so that they use IDOC to send scanned documents to SAP?

As for me how do i receive or pick the scanned documents using idocs. I have read about idocs configuration and a lot more but still not clear to me.

Thank you for your help in advance

Regards

William Mutero