# Task Management - Delta Changes

## MODIFIED Requirements

### Requirement: Task Result Display
The system SHALL provide interactive PDF preview of OCR results with layout preservation on Results and Task Detail pages.

#### Scenario: Results page shows layout-preserving PDF preview
- **WHEN** Results page loads with a completed task
- **THEN** page SHALL fetch PDF from `/api/v2/tasks/{task_id}/download/pdf`
- **AND** page SHALL render PDF using react-pdf PDFViewer component
- **AND** page SHALL NOT show placeholder text "請使用上方下載按鈕..."
- **AND** PDF SHALL display with original document layout preserved
- **AND** PDF SHALL support zoom and page navigation controls

#### Scenario: Task detail page shows PDF preview
- **WHEN** Task Detail page loads for a completed task
- **THEN** page SHALL fetch layout-preserving PDF
- **AND** page SHALL render PDF using PDFViewer component
- **AND** page SHALL NOT show placeholder text
- **AND** PDF SHALL visually match original document layout

#### Scenario: Preview handles loading state
- **WHEN** PDF is being generated or fetched
- **THEN** page SHALL display loading spinner
- **AND** page SHALL show progress indicator during PDF generation
- **AND** page SHALL NOT show error or placeholder text

#### Scenario: Preview handles errors gracefully
- **WHEN** PDF generation fails or file is missing
- **THEN** page SHALL display helpful error message
- **AND** error message SHALL suggest trying download again or contact support
- **AND** page SHALL NOT crash or expose technical errors to user
- **AND** page MAY fallback to markdown preview if PDF unavailable

## ADDED Requirements

### Requirement: Interactive PDF Viewer Features
The PDF viewer component SHALL provide essential viewing controls for user convenience.

#### Scenario: PDF viewer provides zoom controls
- **WHEN** user views PDF preview
- **THEN** viewer SHALL provide zoom in (+) and zoom out (-) buttons
- **AND** viewer SHALL provide fit-to-width option
- **AND** viewer SHALL provide fit-to-page option
- **AND** zoom level SHALL persist during page navigation

#### Scenario: PDF viewer provides page navigation
- **WHEN** PDF contains multiple pages
- **THEN** viewer SHALL display current page number and total pages
- **AND** viewer SHALL provide previous/next page buttons
- **AND** viewer SHALL provide page selector dropdown
- **AND** page navigation SHALL be smooth without flickering

### Requirement: Frontend PDF Library Integration
The frontend SHALL use react-pdf for PDF rendering capabilities.

#### Scenario: react-pdf configured correctly
- **WHEN** application initializes
- **THEN** react-pdf library SHALL be installed and imported
- **AND** PDF.js worker SHALL be configured properly
- **AND** worker path SHALL point to correct pdfjs-dist worker file
- **AND** PDF rendering SHALL work without console errors
