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Custom PDF metadata extractor implementation is not available in the Elixir binding. Document extractors must be implemented in Rust using the `DocumentExtractor` trait.

To implement a custom PDF metadata extractor in Rust:

1. Implement the `Plugin` and `DocumentExtractor` traits
2. Add support for PDF MIME types: `application/pdf`
3. Use a PDF library (e.g., pdfium-render, pdf crate) to extract metadata
4. Register the extractor in the Rust core

See the Rust plugin documentation for implementing custom `DocumentExtractor` plugins for PDF files.