This PR resolves issue #129 by adding an open-source alternative to the PDFTron/PDFNet dependency.

Problem

• PDFTron/PDFNet now requires a license key (no longer free)
• The PDFNetPython3 pip package is not freely available
• Users can't run the existing pdf_compressor.py without a commercial license

Solution

• Added pdf_compressor_ghostscript.py using Ghostscript (open-source)
• Updated README.md to document both methods and installation instructions
• Updated requirements.txt to note system dependencies

Features

• Cross-platform Ghostscript executable detection (gs, gswin64c, gswin32c)
• Quality levels 0-4 mapping to Ghostscript -dPDFSETTINGS (/screen, /ebook, /printer, /prepress, /default)
• Same output format as original script (compression summary)
• No Python dependencies required

Testing

Tested with bert-paper.pdf from the repo:

• Input: 757.00KB
• Compressed (power=1): 407.09KB
• Compression ratio: 46.22%

Usage

# Install Ghostscript (system dependency)
# macOS: brew install ghostscript
# Ubuntu: sudo apt-get install ghostscript  
# Windows: https://ghostscript.com/releases/

# Compress PDF
python pdf_compressor_ghostscript.py bert-paper.pdf compressed.pdf 1

Fixes #129