Is is not yet supported in QBO V3 API service. This feature should be included in future API releases.
Attachment is not same as what you are looking for. But still, you can have a look if it serves your purpose. https://developer.intuit.com/docs/0025_quickbooksapi/0050_data_services/020_key_concepts/attachments
This feature was available in QBO V2( it is deprecated now). https://developer.intuit.com/docs/95_deprecated/qbo_v2/0400_qbo_v2_reference/invoice#Retrieving_an_Invoice_as_PDF
Thanks