Adopt/state policy/licensing for externally-sourced PDFs included as test fixtures

There is no test fixture licensing notice or attribution that I could find in this repo; yet several test fixtures appear to be sourced from "public domain" or potentially copyrighted sources - separate from the MIT License at the repo root.

Please identify and adopt a policy of what fixtures are allowed to be included directly in the repo to test against, or must be fetched by the test harness externally/out-of-band.

No files, tests, or entry points are named. Start by inventorying the test fixtures and their sources, then review the repository's existing licensing materials; done means a documented fixture policy and the required attribution or out-of-band fetching approach are recorded.

Written by the indexing model from the issue text.