Scan PDF Lightweight

Lightweight Scanned PDFs for OCR in .NET

Use this sub-skill when scanned PDFs are too heavy or unstable for OCR upload. The goal is not “compression at any cost”; it is to create an OCR-friendly input PDF while keeping behavior safe through explicit fallback and diagnostics.

When to Use This Skill

Use this skill when:

• Reducing scanned PDF size before sending it to an OCR service
• Stabilizing OCR uploads that fail on large request bodies or weak networks
• Rebuilding scan PDFs as grayscale JPEG-based OCR input in a .NET application
• Keeping PDF preprocessing inside DDD boundaries without leaking image logic into Application
• Adding tests and diagnostics so lightweighting behavior is observable and safe

Related Skills

• dotnet-pdf — Router for PDF-related .NET work
• dotnet-ocr-matching-workflow — End-to-end OCR workflow that can consume lightweighted PDFs
• dotnet-wpf-dify-api-integration — OCR API integration on the client side
• dotnet-wpf-ocr-parameter-input — Future UI surfacing for OCR parameters

Core Principles

1. Optimize Input, Not Business Rules — Lightweighting belongs to Infrastructure; Application asks for OCR-ready chunks and stays orchestration-focused (基礎と型)
2. Fallback Before Regression — If the rebuilt PDF is larger than the original chunk, use the original chunk instead (ニュートラル)
3. TDD Before Tuning — Lock in chunk behavior, fallback, and diagnostics with tests before changing DPI or JPEG quality (継続は力)
4. Observe the Result — Emit diagnostic events that tell operators whether preprocessing was applied or skipped (成長の複利)

Workflow: Add Scan PDF Lightweighting

Step 1 — Define the preprocessing contract

Create an OCR preprocessing settings model in Domain and expose a PDF processor contract that can return OCR-ready chunks. Application should request “OCR chunks,” not describe image-processing steps itself.

Example shape:

public sealed record OcrPreprocessSettings(
    bool Enabled,
    int TargetDpi,
    bool UseGrayscale,
    int JpegQuality);

Why: this keeps policy explicit and testable while preserving DDD boundaries.

Step 2 — Implement rasterize → encode → rebuild in Infrastructure

Render each PDF page to a bitmap, convert to grayscale if needed, encode as JPEG, and rebuild a temporary OCR PDF. In the project implementation, PDFtoImage, SkiaSharp, and PdfSharpCore form that pipeline well.

Typical flow:

1. Split the source PDF into OCR chunks
2. Render each page at target DPI
3. Encode page images as JPEG
4. Rebuild a temporary PDF from those images
5. Compare size against the original chunk

Why: scanned PDFs often carry bulky image payloads, and controlled re-encoding can reduce upload cost substantially.

Step 3 — Add safe fallback logic

If the rebuilt OCR PDF is larger than the original chunk, keep the original chunk instead. Also delete temporary .preprocessed files on both failure and fallback paths.

Why: lightweighting is an optimization, not a correctness requirement. The fallback path protects reliability.

Step 4 — Wire OCR-only usage in Application

Use the OCR-ready chunk method only for the OCR upload path. Do not let lightweighting affect final saved pages or unrelated PDF output unless that is explicitly part of the feature.

Why: OCR input optimization and final artifact preservation are different concerns.

Step 5 — Emit diagnostics

Publish a structured diagnostic result that reports:

• chunk index
• page range
• original vs output size
• DPI
• grayscale on/off
• whether preprocessing was applied or skipped

Why: operators need to confirm what actually happened when debugging OCR upload issues.

Step 6 — Validate with TDD

Cover at least these tests:

• OCR chunk path uses lightweight preprocessing
• fallback occurs when rebuilt PDF grows
• grayscale output is actually grayscale
• diagnostics report preprocessing result
• final saved single-page extraction path is unchanged

Why: PDF/image changes are easy to “make work once” and hard to keep safe without regression coverage.

Best Practices

• Prefer explicit settings objects over scattered magic numbers
• Keep image-processing libraries isolated in Infrastructure
• Record preprocessing outcome in structured diagnostics, not only in message strings
• Tune DPI and JPEG quality with a real sample PDF, not guesswork

Pitfalls

• Changing final output artifacts unintentionally: Lightweighting for OCR should not silently alter the later page-save flow.
• Treating smaller size as always better: Smaller PDFs can still hurt OCR quality; validate against real scan samples.
• Skipping cleanup of temporary files: Rebuilt PDF temp files can accumulate quickly during failures.
• Baking project-specific constants into the skill title: Keep the trigger reusable even if the first implementation came from one project.