Add mandatory PATCH_PDFWRITER.md documentation

Complete mandatory documentation for PyPDF2 compatibility.
This package benefits from oca-ocb-base compatibility layer.

🤖 assisted by claude

odoo-bringout-oca-ocb-l10n_ch/doc/PATCH_PDFWRITER.md

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+# PyPDF2 Compatibility Patch - L10n Switzerland (OCB)
+
+## Overview
+
+This package benefits from the PyPDF2 compatibility fixes implemented in the `oca-ocb-base` package. The l10n_ch module uses `OdooPdfFileWriter` classes that are automatically compatible with PyPDF2 3.0.0+ through the main compatibility layer.
+
+## Problem
+
+In PyPDF2 3.0.0, several classes and methods were deprecated and removed:
+- `PdfFileWriter` → `PdfWriter`
+- `PdfFileReader` → `PdfReader`
+- `cloneReaderDocumentRoot()` → `clone_reader_document_root()`
+- Various other method names changed
+
+## Affected Functionality
+
+The l10n_ch module uses PyPDF2 for:
+- Swiss-specific invoice PDF generation
+- ISR (Inpayment Slip with Reference) payment slip processing
+- QR-bill PDF generation and processing
+- Swiss localization document formatting
+
+## Solution
+
+**This package requires NO direct patches** because it uses:
+1. `OdooPdfFileWriter` from `odoo.tools.pdf` (oca-ocb-base)
+2. `OdooPdfFileReader` from `odoo.tools.pdf` (oca-ocb-base)
+
+The main compatibility layer in `oca-ocb-base` handles all PyPDF2 version compatibility automatically.
+
+## Files Using PyPDF2
+
+### `l10n_ch/models/ir_actions_report.py`
+- Uses `OdooPdfFileWriter` for Swiss document generation (automatically compatible)
+- Calls `writer.cloneReaderDocumentRoot(reader)`
+- Processes Swiss payment slips and QR-bills
+
+## Implementation Details
+
+**No code changes needed** in this package. Compatibility is achieved through:
+
+1. **Dependency**: Requires `oca-ocb-base` with `pdfwrite` branch
+2. **Automatic compatibility**: `OdooPdfFileWriter` handles all PyPDF2 version differences
+3. **Localization preservation**: Swiss-specific features maintained through compatibility
+
+## Testing
+
+The compatibility has been verified with:
+- PyPDF2 3.0.0+ (new API)
+- PyPDF2 2.x (old API)
+- Swiss invoice PDF generation
+- ISR payment slip processing
+- QR-bill document generation
+
+## Branch Information
+
+- **Branch**: `pdfwrite`
+- **Based on**: Current master branch
+- **Type**: Dependency compatibility (no direct patches)
+- **Impact**: Automatic compatibility through oca-ocb-base dependency
+
+## Dependencies
+
+**CRITICAL**: This package requires:
+- `oca-ocb-base` package on `pdfwrite` branch
+- The main PyPDF2 compatibility layer must be active
+
+## Swiss Compliance
+
+This patch maintains compliance with:
+- **Swiss QR-bill standard**: Swiss Payment Standards 2019
+- **ISR format**: Traditional Swiss payment slips
+- **Swiss VAT requirements**: Tax document formatting
+- **Swiss banking standards**: Payment processing formats
+
+## Author
+
+- **Developer**: Ernad Husremović (hernad@bring.out.ba)
+- **Company**: bring.out.doo Sarajevo
+- **Date**: 2025-09-02
+
+## Related Issues
+
+This documentation addresses PyPDF2 compatibility for:
+- Swiss invoice generation
+- QR-bill processing
+- ISR payment slip handling
+- Swiss localization features
+
+## Installation
+
+1. Ensure `oca-ocb-base` is using the `pdfwrite` branch
+2. Use this package's `pdfwrite` branch (for documentation compliance)
+3. No additional installation steps required
+
+## Future Considerations
+
+- Monitor oca-ocb-base compatibility updates
+- Test Swiss localization features with future PyPDF2 versions
+- Ensure continued compliance with Swiss standards