CheahPDF - PDF Color Inverter

Description: Free local Flask app converts PDF white backgrounds to black backgrounds with white text. Dark mode for PDFs. MIT licensed, runs locally, 1000 DPI output.

A simple Flask web application that converts PDF files from white background to black background with white text at high resolution (1000 DPI).

Features

• Upload PDF files via web interface
• Convert white backgrounds to black
• Convert black text to white
• High-resolution output at 1000 DPI
• Automatic file download after processing
• Detailed console logging for debugging

Project Structure

cheahpdf/
├── app.py                 # Main Flask application
├── pdf_processor.py       # PDF color inversion logic
├── .env                   # Environment configuration
├── pyproject.toml         # Python project configuration for uv
├── requirements.txt       # Python dependencies (legacy)
├── templates/
│   └── index.html        # Web upload interface
├── uploads/              # Temporary uploaded files (auto-created)
├── outputs/              # Processed PDF files (auto-created)
└── README.md             # This file

Dependencies

• Flask - Web framework
• PyMuPDF (fitz) - PDF processing library
• Pillow (PIL) - Image processing for color inversion
• python-dotenv - Environment variable loading

Installation & Setup

Using uv (recommended)

# Install dependencies
uv sync

# Run the application
uv run python app.py

Using pip (alternative)

# Install dependencies
pip install -r requirements.txt

# Run the application
python app.py

Configuration

Edit .env file to customize settings:

FLASK_ENV=development
FLASK_DEBUG=True
UPLOAD_FOLDER=uploads
OUTPUT_FOLDER=outputs
MAX_FILE_SIZE=52428800     # 50MB in bytes
PDF_DPI=1000               # Output resolution

Usage

1. Start the application: uv run python app.py
2. Open browser to http://localhost:8002
3. Upload a PDF file
4. Wait for processing (console shows progress)
5. Download automatically starts when complete

How It Works

1. Upload: PDF file is uploaded to uploads/ directory
2. Processing: Each page is:
   • Rendered to high-resolution image (1000 DPI)
   • Converted to PIL Image
   • Colors inverted using ImageOps.invert()
   • Converted back to PDF page
3. Output: Inverted PDF saved to outputs/ directory
4. Download: File sent to user, temporary files cleaned up

Technical Details

• Port: 8002 (avoids macOS Control Center conflicts on 5000)
• File Limits: 50MB max upload size
• Supported Formats: PDF only
• Processing: Synchronous (user waits for completion)
• Cleanup: Temporary files automatically removed

Logging

Detailed console logs show:

• File upload status
• Processing progress per page
• Error details with stack traces
• File sizes and processing times

Error Handling

• Input validation for file type and size
• Comprehensive error logging
• User-friendly error messages
• Automatic cleanup on failures

Development

The application uses Flask's debug mode for development:

• Auto-reload on code changes
• Detailed error pages
• Debug console available

License

MIT License - see LICENSE file for details.