Download academic paper by DOI, arXiv ID, or URL.

Prioritizes open access sources (Unpaywall, arXiv, CORE) before Sci-Hub.
Sources: Unpaywall (OA), arXiv (OA), CORE (OA), Sci-Hub (last resort)

Args:
    identifier: DOI, arXiv ID, or URL
    output_dir: Save directory (default: './downloads')

Returns:
    Markdown with file path, metadata, source, or error message

Examples:
    paper_download("10.1038/nature12373")  # DOI
    paper_download("2301.00001")  # arXiv ID
    paper_download("https://arxiv.org/abs/2301.00001")  # URL

Download multiple papers sequentially (1-50 max, 2s delay).

Prioritizes open access sources (Unpaywall, arXiv, CORE) before Sci-Hub.

Args:
    identifiers: List of DOIs, arXiv IDs, or URLs
    output_dir: Save directory (default: './downloads')

Returns:
    Markdown summary with statistics, successes, and failures

Examples:
    paper_batch_download(["10.1038/nature12373", "2301.00001"])
    paper_batch_download(dois, "/papers")

Get paper metadata without downloading (fast, <1s).

Sources: Unpaywall, Crossref, arXiv APIs
Returns: title, authors, year, journal, OA status, available sources

Args:
    identifier: DOI, arXiv ID, or URL

Returns:
    JSON with metadata fields

Examples:
    paper_metadata("10.1038/nature12373")  # DOI
    paper_metadata("2301.00001")  # arXiv ID