Configure GraphQL
Configure API With GraphQL Protocol, and Publish it.

GraphQL Report Query
For GraphQL Report Query User "ReportData" in query for fetch report data.
before making API calls please make sure you have provided correct authentication parameter.
Provide GraphQL Query as per your requirement considering below example.
query MyQery {
ReportData(
converter: "pdf"
report_ref: "sale.report_saleorder"
res_ids: [4,5]
data: {}
)
}
Now, in result you will get query result with "file_data" which is base64 data of file.