# PDF Viewer

## Objective

Let people read a PDF inside the app instead of downloading it first.

An in-app PDF reader with page navigation, zoom, rotation, and text search over the document.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Render pages inside the app's existing layout with page navigation, zoom, rotation, and a search over the document text. A download button stays available at all times.
2. Serve the file through the app's existing Secure File Downloads path so viewing is authorised exactly like downloading, and a viewer URL cannot be shared to bypass permissions.
3. Reserve each page's dimensions before it renders so the scroll position stays stable and the document does not jump while the reader scrolls.
4. Render only the pages near the viewport and discard those far outside it, so a long document does not grow memory until the tab dies.
5. Do not build this next to the app's PDF Export as a separate document stack. Viewing and exporting should agree on where files live, how they are named, and who may read them.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- A four-hundred-page file must never render all at once. Render lazily around the viewport and keep the page count and jump-to-page control working before every page has been drawn.
- Password-protected files need a prompt, and corrupt or truncated files need a plain message with a download link. Neither may present as an endless spinner.
- Text must stay selectable and searchable. Rendering every page to a flat image loses copy, search, and screen reader access, which is most of why people wanted the viewer.
- Zoom level, current page, and rotation should be remembered per document, so returning to a long file does not mean scrolling back to where the reader was.
- Low-memory and unsupported devices must fall back to a download or the browser's own viewer rather than crashing the tab.
- PDFs can carry embedded scripts, forms, and remote resources. Render with scripting disabled and do not let the document fetch anything from a third-party host.
- Keyboard users need page navigation, zoom, and search without a mouse, and the page container needs a focusable, scrollable region.
- Printing from inside the viewer should hand off to the real file rather than printing rasterised canvases at screen resolution.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] A PDF opens and is readable in the app without downloading it.
- [ ] Long documents render lazily and remain responsive with hundreds of pages.
- [ ] Text is selectable and the in-document search finds it.
- [ ] Password-protected and corrupt files produce a clear message rather than a hang.
- [ ] Zoom, page, and rotation are restored when the reader returns to a document.
- [ ] Unsupported or low-memory devices degrade to a download link.
- [ ] Access is authorised on every request through the app's existing secure file path.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.