Advanced PDF Cropping Tool

1. Advanced PDF Preview:
   • Uses PDF.js to render PDFs directly in the browser
   • Responsive display that adjusts to container size
   • Page selection dropdown for multi-page PDFs
2. Multiple Crop Modes:
   • Manual selection with draggable/resizable crop area
   • Auto-detect content (simplified version)
   • Custom margin controls
3. Unit Conversion:
   • Supports pixels, inches, centimeters, and millimeters
   • Real-time conversion between units
4. Preview Functionality:
   • Visual preview of the cropped area before final processing
   • Download option for the cropped PDF
5. Robust Error Handling:
   • File type validation (PDF only)
   • File size limit (10MB)
   • Error messages for various failure scenarios
6. Responsive Design:
   • Works on both desktop and mobile devices
   • Touch support for crop area manipulation
   • Adapts layout for smaller screens
7. User-Friendly Interface:
   • Visual feedback during operations
   • Loading indicators for long-running tasks
   • Clear instructions and controls

Implementation Notes

1. This solution requires the following libraries:
   • PDF.js for PDF rendering
   • PDF-lib for actual PDF cropping
   • FileSaver.js for downloading files
   • Bootstrap for responsive layout and styling
2. For a production environment, you might want to:
   • Host the PDF.js worker locally
   • Implement server-side cropping for very large files
   • Add more sophisticated content detection
   • Implement user authentication if needed
3. The auto-detect content feature is simplified in this example. A real implementation might use more advanced algorithms to detect page content boundaries.