Stirling

OCR (Optical Character Recognition)

Make scanned PDFs searchable and selectable by recognizing text in images. Uses the Tesseract OCR engine.

Requires a server backend

OCR needs a server-side backend with Tesseract installed. The desktop app cannot OCR in local-only mode - connect it to Stirling Cloud or a self-hosted server that has OCR available.

When You Need OCR#

  • Scanned paper documents (no text layer)
  • Photos of documents or whiteboards
  • Image-only PDFs where you can't select or search text

How to Use#

  1. Upload Your PDF - Select a scanned or image-based PDF
  2. Select Language(s) - Choose the language(s) in your document
  3. Configure Options - Adjust OCR mode and preprocessing (optional)
  4. Process - Run OCR
  5. Download - Get your searchable PDF

Options#

Option Values Description
Languages Select from installed packs Must match the languages in your document. Select multiple if needed
OCR Mode Auto (default), Force, Strict Auto skips pages that already have text. Force re-OCRs everything. Strict aborts if any text is found
Compatibility Mode On/Off Uses sandwich PDF format for better compatibility with older software (larger files)

Advanced Options#

Option Description
Deskew Automatically straighten tilted/skewed pages
Clean Input Preprocess by removing noise and enhancing contrast for better recognition
Clean Final Output Post-process the final PDF to remove OCR artifacts
Create Text File Generate a separate .txt file with the extracted text (output as ZIP)

Advanced options require OCRmyPDF. With Tesseract only, they are ignored.

Language Packs#

Available languages depend on which Tesseract language packs are installed. The default Docker image includes English, German, French, Portuguese, and Chinese Simplified. To add more languages, see the OCR Configuration Guide.

Limitations#

  • Tesseract recognizes text only - it does not do table-structure or formula recognition
  • Handwritten text has limited accuracy
  • Stylized/decorative fonts and very small text (< 8pt) are challenging
  • For best results, use 300 DPI or higher scans with good contrast
  • To OCR non-English documents, install the matching Tesseract language pack (see Language Packs)

API Usage#

bash
curl -X POST http://stirling-pdf:8080/api/v1/misc/ocr-pdf \
  -F "[email protected]" \
  -F "languages=eng" \
  -F "languages=spa" \
  -F "ocrType=skip-text" \
  -F "ocrRenderType=hocr" \
  -F "deskew=true" \
  -F "clean=true" \
  -F "cleanFinal=true" \
  -F "sidecar=false" \
  -o searchable.pdf

See API Documentation for complete endpoint reference.

  • Convert - Convert OCR'd PDFs to Word, text, or other formats
  • Compress - Reduce file size after OCR
  • Auto-Rename - Rename files based on OCR'd content
Edit this page on GitHub