# PDF Server

![Screenshot](screenshot.png)

A simple interactive PDF viewer that uses [PDF.js](https://mozilla.github.io/pdf.js/). Launch it w/ a few PDF files and/or URLs as CLI args (+ support loading any additional pdf from arxiv.org).

## What This Example Demonstrates

### 1. Chunked Data Through Size-Limited Tool Calls

On some host platforms, tool calls have size limits, so large PDFs cannot be sent in a single response. This example shows a possible workaround:

**Server side** (`pdf-loader.ts`):

```typescript
// Returns chunks with pagination metadata
async function loadPdfBytesChunk(entry, offset, byteCount) {
  return {
    bytes: base64Chunk,
    offset,
    byteCount,
    totalBytes,
    hasMore: offset + byteCount < totalBytes,
  };
}
```

**Client side** (`mcp-app.ts`):

```typescript
// Load in chunks with progress
while (hasMore) {
  const chunk = await app.callServerTool("read_pdf_bytes", { pdfId, offset });
  chunks.push(base64ToBytes(chunk.bytes));
  offset += chunk.byteCount;
  hasMore = chunk.hasMore;
  updateProgress(offset, chunk.totalBytes);
}
```

### 2. Model Context Updates

The viewer keeps the model informed about what the user is seeing:

```typescript
app.updateModelContext({
  structuredContent: {
    title: pdfTitle,
    currentPage,
    totalPages,
    pageText: pageText.slice(0, 5000),
    selection: selectedText ? { text, start, end } : undefined,
  },
});
```

This enables the model to answer questions about the current page or selected text.

### 3. Display Modes: Fullscreen vs Inline

- **Inline mode**: App requests height changes to fit content
- **Fullscreen mode**: App fills the screen with internal scrolling

```typescript
// Request fullscreen
app.requestDisplayMode({ mode: "fullscreen" });

// Listen for mode changes
app.ondisplaymodechange = (mode) => {
  if (mode === "fullscreen") enableScrolling();
  else disableScrolling();
};
```

### 4. External Links (openLink)

The viewer demonstrates opening external links (e.g., to the original arxiv page):

```typescript
titleEl.onclick = () => app.openLink(sourceUrl);
```

## Usage

```bash
# Default: loads a sample arxiv paper
bun examples/pdf-server/server.ts

# Load local files (converted to file:// URLs)
bun examples/pdf-server/server.ts ./docs/paper.pdf /path/to/thesis.pdf

# Load from URLs
bun examples/pdf-server/server.ts https://arxiv.org/pdf/2401.00001.pdf

# Mix local and remote
bun examples/pdf-server/server.ts ./local.pdf https://arxiv.org/pdf/2401.00001.pdf

# stdio mode for MCP clients
bun examples/pdf-server/server.ts --stdio ./papers/
```

**Security**: Dynamic URLs (via `view_pdf` tool) are restricted to arxiv.org. Local files must be in the initial list.

## Tools

| Tool             | Visibility | Purpose                            |
| ---------------- | ---------- | ---------------------------------- |
| `list_pdfs`      | Model      | List indexed PDFs                  |
| `display_pdf`    | Model + UI | Display interactive viewer in chat |
| `read_pdf_bytes` | App only   | Chunked binary loading             |

## Architecture

```
server.ts           # MCP server (233 lines)
├── src/
│   ├── types.ts        # Zod schemas (75 lines)
│   ├── pdf-indexer.ts  # URL-based indexing (44 lines)
│   ├── pdf-loader.ts   # Chunked loading (171 lines)
│   └── mcp-app.ts      # Interactive viewer UI
```

## Key Patterns Shown

| Pattern           | Implementation                           |
| ----------------- | ---------------------------------------- |
| App-only tools    | `_meta: { ui: { visibility: ["app"] } }` |
| Chunked responses | `hasMore` + `offset` pagination          |
| Model context     | `app.updateModelContext()`               |
| Display modes     | `app.requestDisplayMode()`               |
| External links    | `app.openLink()`                         |
| Size negotiation  | `app.sendSizeChanged()`                  |

## Dependencies

- `pdfjs-dist`: PDF rendering
- `@modelcontextprotocol/ext-apps`: MCP Apps SDK