Pure-Ruby (no external system libraries) invoice-PDF generator, usable as a library, CLI, or server: A4/US Letter formats, purchaser/provider info boxes with addresses and tax IDs, SWIFT/IBAN payment details, itemized tables (description/quantity/price/tax/totals), automatic page numbering, customizable labels, custom fonts/logos/backgrounds/stamps/signatures/QR codes, and JSON-driven invoice data — fast enough for real-time request handling.