OCR PDF – Frequently Asked Questions

The OCR PDF tool uses optical character recognition to extract text from scanned PDFs and images, making them searchable and editable.

Key Features:

• OCR.space API: Cloud-based text recognition service
• 21 Languages: Support for multiple languages including English, Spanish, French, Chinese, Arabic, and more
• Searchable PDF: Create PDFs with invisible text overlay for search and selection
• Text-Only PDF: Extract text to plain PDF without images
• Session Storage: Cache OCR results to avoid re-processing
• High Accuracy: Professional OCR engine for reliable text extraction

Technical Implementation: The tool integrates with OCR.space API, a cloud-based OCR service. Users upload scanned PDFs, select language (from 21 options), and choose output type. The PDF is sent to OCR.space API which returns extracted text with coordinates. For searchable PDFs, text is overlaid invisibly on original images using FPDI/TCPDF. For text-only PDFs, only extracted text is included. OCR results are stored in session storage to avoid re-processing the same file.

OCR.space is a cloud-based optical character recognition API service.

What is OCR.space?

• Type: Cloud-based OCR API service
• Website: ocr.space
• Purpose: Text recognition from images and PDFs
• Pricing: Free tier (limited) + paid plans for higher volume

How It Works:

1. API Integration: PDF Lab connects to OCR.space API
2. File Upload: PDF sent to OCR.space servers via API
3. Text Recognition: OCR.space processes PDF with advanced OCR engines
4. Results Returned: API returns extracted text with word coordinates
5. PDF Generation: PDF Lab creates searchable or text-only PDF from results

OCR.space Features:

• 21 Languages: Multi-language support
• High Accuracy: Advanced OCR algorithms
• Word Coordinates: Provides position data for text overlay
• Image Preprocessing: Automatic image enhancement for better OCR

API Key Required:

• Free API key available at ocr.space
• Free tier includes limited OCR requests per month
• Paid plans for higher volume usage

OCR.space supports 21 different languages for text recognition.

Supported Languages Include:

• Western European: English, Spanish, French, German, Italian, Portuguese, Dutch
• Eastern European: Russian, Polish, Czech
• Asian: Chinese (Simplified), Chinese (Traditional), Japanese, Korean
• Middle Eastern: Arabic, Hebrew
• Nordic: Danish, Finnish, Norwegian, Swedish
• Other: Turkish, Greek

Why Language Selection Matters:

• Accuracy: OCR engines are trained on specific languages
• Character Sets: Different languages use different alphabets (Latin, Cyrillic, Chinese characters, Arabic script)
• Recognition Quality: Selecting correct language improves accuracy by 10-20%
• Special Characters: Language-specific diacritics and symbols recognized correctly

How to Choose Language:

• Select the language of the text in your PDF before processing
• For multi-language documents, select the primary language
• English is the default and works well for general use

A searchable PDF combines original scanned images with invisible text overlay, making the PDF searchable and selectable.

How Searchable PDF Works:

• Visual Layer: Original scanned image displayed normally
• Text Layer: OCR-extracted text placed invisibly behind/over the image
• Coordinate Matching: Text positioned at exact coordinates matching image text
• Result: PDF looks identical to scan, but text is selectable and searchable

Searchable PDF Features:

• Search (Ctrl+F): Find text using PDF reader's search function
• Text Selection: Click and drag to select text
• Copy Text: Copy extracted text to clipboard
• Screen Readers: Accessible to screen readers for visually impaired users
• Visual Preservation: Looks exactly like the original scan

Use Cases:

• Scanned contracts, legal documents (need original appearance + searchability)
• Historical documents, archival scans
• Books, articles (maintain original formatting while allowing search)

File Size:

• Similar to original scanned PDF (images retained)
• Text layer adds minimal size

A text-only PDF contains only the extracted text without the original scanned images.

How Text-Only PDF Works:

• Text Extraction: OCR extracts all text from scanned PDF
• Image Removal: Original scanned images are discarded
• Text Document: New PDF created with just the extracted text
• Formatting: Basic text formatting (paragraphs, line breaks)

Text-Only PDF Features:

• Small File Size: Much smaller than scanned PDF (no images)
• Fully Editable: Text can be edited in PDF editors
• Searchable: All text is searchable by default
• Copyable: All text can be selected and copied
• Plain Appearance: Loses original formatting/appearance

Use Cases:

• Extract text for editing or reformatting
• Reduce file size dramatically
• Create editable versions of scanned documents
• Content analysis, data extraction

File Size Comparison:

• Original Scan: 5 MB (with images)
• Searchable PDF: 5.1 MB (images + text layer)
• Text-Only PDF: 50 KB (text only, 100x smaller!)

Session storage caches OCR results to avoid re-processing the same PDF.

How Session Storage Works:

1. First OCR: User uploads PDF, OCR.space API processes it
2. Store Results: Extracted text saved to browser session storage
   • Key: PDF filename or hash
   • Value: OCR text and coordinates (JSON)
3. Subsequent Upload: If same PDF uploaded again in same session
4. Check Storage: Tool checks if OCR results exist in session storage
5. Instant Retrieval: If found, use cached results (no API call)
6. Time Saved: No waiting for OCR processing again

Benefits:

• Faster Processing: Instant results for repeated files
• API Quota Savings: Doesn't consume OCR.space API credits
• User Experience: Seamless when reprocessing same file

Session Storage Lifespan:

• Duration: Lasts for current browser tab/session only
• Cleared: Automatically deleted when browser tab closes
• Not Persistent: Does not survive browser restart

Use Case Example:

• Upload scanned contract → OCR processes (30 seconds)
• Download searchable PDF
• Realize you want text-only PDF instead
• Re-upload same file, select text-only → instant (from cache, 1 second)

OCR accuracy depends on multiple factors related to the source document quality.

Factors Affecting Accuracy:

Image Quality (Most Important)

• High Resolution (300+ DPI): 95-99% accuracy
• Medium Resolution (150-300 DPI): 85-95% accuracy
• Low Resolution (<150 DPI): 60-85% accuracy

Text Clarity

• Clear, Dark Text on White Background: Best accuracy
• Good Contrast: Black on white, high contrast colors
• Poor Contrast: Light gray on white, faded text → lower accuracy

Document Type

• Printed Text: 95-99% accuracy (typed/printed documents)
• Clean Scans: 90-98% accuracy
• Handwriting: 60-80% accuracy (OCR struggles with cursive)
• Degraded Documents: Faded, stained, or damaged → lower accuracy

Language Selection

• Correct Language: 10-20% improvement over wrong language
• English Default: Works well for Latin alphabet languages
• Specialized Languages: Chinese, Arabic, Japanese require correct selection

Tips for Best Accuracy:

• Scan at 300 DPI or higher
• Ensure good lighting and contrast
• Use clean, undamaged documents
• Select correct language before OCR
• Use printed text when possible (not handwritten)

Yes! The OCR PDF tool is fully responsive and mobile-friendly.

Mobile Features:

• File Upload: Upload scanned PDFs from mobile device storage
• Language Selection: Touch-friendly dropdown for language choice
• Output Type: Choose searchable PDF or text-only PDF
• Server Processing: All OCR happens via API (no mobile performance impact)
• Download: OCR-processed PDF downloads to device

Mobile Tips:

• Works on iOS (iPhone, iPad) and Android devices
• OCR processing time same as desktop (cloud-based)
• Best used with WiFi for large PDF uploads

No, we do not permanently store your PDF files or OCR results.

How We Handle Your Files:

• Temporary Storage: Uploaded PDFs stored in /tmp folder only during processing
• OCR.space Processing: PDF sent to OCR.space API securely
  • OCR.space also does not store files permanently
• OCR Results (Client-Side): Stored in browser session storage only
• Automatic Cleanup: All temporary files deleted after download
• Session Isolation: Each user's files isolated with unique identifiers

Privacy Guarantee:

• We do not access or analyze file contents
• We do not permanently store documents or OCR results
• OCR.space follows strict privacy policies
• Session storage is local to your browser

The OCR PDF tool combines cloud OCR services with PDF generation libraries.

OCR Service:

• OCR.space API: Cloud-based text recognition
  • RESTful API integration
  • PDF uploaded to API endpoint
  • Returns JSON with extracted text and coordinates
• 21 Language Support: Language-specific OCR engines
• API Key: Required for authentication (free tier available)

PDF Generation:

• Searchable PDF:
  • FPDI imports original scanned pages
  • TCPDF adds invisible text layer using OCR coordinates
  • Text() method places text at specified positions
  • Text color set to transparent or white for invisibility
• Text-Only PDF:
  • TCPDF creates new PDF with just extracted text
  • Basic formatting (paragraphs, line breaks)
  • No images included

Frontend Technologies:

• Session Storage API: Cache OCR results
  • sessionStorage.setItem('ocr_filename', results)
  • sessionStorage.getItem('ocr_filename')
• JavaScript: File upload, language selection, output type selection
• Base64 Encoding: PDF converted to base64 for API transmission

Processing Workflow:

1. User uploads scanned PDF and selects language + output type
2. Check session storage for cached OCR results
3. If cached, use stored results; otherwise:
   • Convert PDF to base64
   • Send to OCR.space API with language parameter
   • API returns extracted text + coordinates
   • Store results in session storage
4. Generate output PDF:
   • Searchable: Import pages, overlay text invisibly
   • Text-Only: Create PDF with text only
5. Return PDF for download