Short answer: choose staged vector retrieval first for a privacy-focused personal knowledge manager, with one collection contract, bounded metadata filters, and source citations; add hybrid retrieval only after representative PDF tests show a specific lexical recall gap.

For a marketplace team answering questions over a private folder of seller agreements, policy PDFs, and reconciliation manuals, this is an index-cost decision as much as a relevance decision. A second index is another body of data to ingest, refresh, authorize, reconcile, and eventually delete. It should therefore earn its place with evidence rather than arrive as an architectural reflex.

The decision is deliberately conditional. Vector-first is appropriate when semantic questions dominate and the team can evaluate retrieval against representative documents.…