For scanned invoices, the requirement is structured field extraction (invoice number/ID, vendor, totals) from document images or PDFs at scale. The best fit is Azure Document Intelligence because it is purpose-built for document processing and provides prebuilt invoice models that combine OCR with layout/structure understanding to extract common invoice fields into a structured output. Microsoft's invoice model is explicitly designed to analyze invoices (including scanned images) and return key fields and line items in structured form, which directly maps to this scenario.

Azure Vision (B) can perform OCR and basic image analysis, but OCR alone typically returns text without robust invoice-specific field interpretation (e.g., reliably identifying ''Invoice ID'' vs. ''Order ID,'' totals vs. subtotals, vendor vs. ship-to). Document Intelligence is optimized for advanced document structure extraction and is therefore the ''best'' single recommendation.

Azure AI Search (C) focuses on indexing and retrieval/knowledge mining across a corpus; it's not the primary service for extracting invoice fields for downstream processing. Azure Machine Learning (D) could be used to build a custom model, but that adds cost and time compared with a prebuilt invoice extractor designed for this document type.