BabelDOC is a production-grade PDF translation library and CLI tool that translates documents while preserving original layout, fonts, formulas, and styles. Its core architecture revolves around a custom Intermediate Representation (IL) that decouples PDF parsing from rendering, enabling a modular pipeline of parsing, layout analysis, LLM-based translation, typesetting, and PDF generation. The system supports complex PDF features including CJK text, Type3 fonts, tables, and formulas, and offers both monolingual and dual-language output modes. The translation pipeline is driven by an OpenAI-compatible LLM, with sophisticated prompt engineering for context-aware paragraph translation, glossary integration, and formula/style preservation. Layout analysis uses a DocLayout-YOLO ONNX model for detecting text blocks, tables, and figures. The codebase features a dual parser architecture, retaining a modified pdfminer for compatibility while developing a new native parser with improved font handling and security (CMap pickle deserialization hardening). BabelDOC demonstrates strong software engineering with several genuinely reusable components, including its typed IL data model, a priority thread pool executor, cross-platform memory monitoring utilities, and a high-performance glossary matching system using Hyperscan. The project is actively maintained with frequent releases, comprehensive CI/CD, and responsive security patching. While its complexity and dual-parser architecture present maintenance challenges, the overall code quality, architectural decisions, and reusable patterns make it a valuable reference for PDF processing and LLM-integrated document workflows.