# Change: Unify Direct Track PDF Rendering with Background Image + Invisible Text Layer

## Why

Direct Track PDF generation currently has visual rendering issues:
1. **Chart text overlap**: Text elements extracted from PDF text layer (e.g., "Temperature, °C") overlap with chart images
2. **Z-order problems**: White text on dark backgrounds becomes invisible when rendered incorrectly
3. **Office document issues**: PPT/DOC/XLS converted PDFs lose visual fidelity (vector graphics, gradients)

The root cause is that Direct Track tries to render individual elements (text, images, tables) separately, which leads to z-order conflicts and missing visual content.

## What Changes

### Backend Changes

1. **Unified Background Image Rendering for All Direct Track**
   - Render source PDF page as full-page background image (2x resolution)
   - Draw invisible text layer on top (PDF Text Rendering Mode 3)
   - Text remains searchable/extractable but doesn't visually overlap

2. **Chart Region Exclusion**
   - Add `CHART` element type to `regions_to_avoid`
   - Chart-internal text (axis labels, legends) will NOT be in invisible text layer
   - These texts are already visible in the background image and don't need translation

3. **Skip Element Rendering When Background Exists**
   - When background image is rendered, skip individual image/table rendering
   - Only draw invisible text layer for searchability and translation extraction

### Frontend Considerations

1. **No UI Changes Required for Layout PDF**
   - Layout PDF generation is automatic, no user options needed
   - Visual output will match source PDF exactly

2. **Translation Flow Clarification**
   - Layout PDF: Background image + invisible text (for preview)
   - Translated PDF: Reflow layout with real visible text (page-by-page)
   - Chart text excluded from translation (already in background image)

## Impact

- **Affected specs**: document-processing, result-export, translation
- **Affected code**:
  - `backend/app/services/pdf_generator_service.py` (main changes)
  - `backend/app/services/direct_extraction_engine.py` (chart detection)
- **File size**: Output PDF will be larger due to embedded page images (~2MB per page at 2x resolution)
- **Processing time**: Slight increase for page rendering

## Migration

- No database changes required
- No API changes required
- Existing tasks can be re-exported with new PDF generation logic