Pdfrb is a pure-Ruby PDF library: a byte-level reader, an Arlington-model-driven typed domain model, and a serializer. The PDF object model is sourced directly from the vendored Arlington PDF Model TSVs (machine-readable ISO 32000-2:2020), so field metadata, version predicates, and validators stay aligned with the spec by data, not by hand-coded mimicry. Two-direction contract: "PDF file <=> Model" and "API Builder Input => Model". Mirrors the layered design of the sibling postscript gem.