Error when using document data for IPP send Document Request

[hungnq39]

After Register Printer with Universal Print Service, I build a IPP Proxy App as a connector to connect Output Device with Infrastructure Printer. 
After implement Fetch Document IPP Operation , UP Services response document data. But if I use this data to Send-Document Operation to Output  Device. This data is not type Buffer. It return type string.
I see Graph API have a api return same data. It shown below 
https://docs.microsoft.com/en-us/graph/api/printdocument-get-file?view=graph-rest-beta 

[#2]

Hello @hungnq39 the print payload is binary data of the remaining HTTP payload that immediately follows the document attributes.    
Please see section 5.5.2 of the IPP INFRA spec: http://ftp.pwg.org/pub/pwg/candidates/cs-ippinfra10-20150619-5100.18.pdf.