# Unpaywall + pdftotext — Design

Two improvements to the paper CLI: try free open-access sources before piracy, and try simple PDF text extraction before heavy ML OCR.

## Download Pipeline

Priority chain for fetching the PDF:

1. **Unpaywall** (new) — hit `https://api.unpaywall.org/v2/{doi}?email={email}`, check `best_oa_location.url_for_pdf`, download if present
2. **LibGen** (existing)
3. **Anna's Archive** (existing)

Unpaywall requires an email address (no API key). Read from `UNPAYWALL_EMAIL` env var. If unset, skip silently — same pattern as `ANNAS_ARCHIVE_KEY`.

## Conversion Pipeline

Priority chain for PDF-to-markdown:

1. **pdftotext** (new) — shell out to `pdftotext -layout <pdf> -`, check output quality
2. **marker-pdf** (existing) — heavy ML OCR fallback

### Quality heuristic for pdftotext output

- Length > 500 characters
- >80% printable ASCII / common unicode (letters, digits, punctuation, whitespace)

If either check fails, or pdftotext isn't on PATH, fall back to marker.

## Nix changes

Add `poppler_utils` to the flake for `pdftotext`. Include in both the dev shell and the wrapped binary PATH.

## Dependencies

No new Rust crate dependencies — `reqwest` and `serde_json` already handle the Unpaywall API call. `pdftotext` is an external binary like `marker_single`.