# Custom PDF backend

> Swap the bundled jspdf pipeline for your own renderer - or run generation in a Worker.

Source: https://docs.docmosaic.com/docs/recipes/custom-pdf-backend

`Editor.Root` accepts a `pdf` prop with two optional functions: `generate` and `estimate`. Anything omitted falls back to the bundled `@docmosaic/core` implementation.

## Why override

-   **Different renderer.** Swap `jspdf` for `pdf-lib`, `pdfkit`, or `react-pdf`.
-   **Worker offload.** Move the synchronous part of generation off the main thread.
-   **Server-side rendering.** Send the document to your backend and stream the result down.
-   **Testing.** Mock the pipeline entirely with deterministic output.

## Signatures

The two functions are typed off the core exports - copy the signatures to stay compatible:

```ts
import type { generatePDF, estimatePDFSize } from '@docmosaic/core';

const generate: typeof generatePDF = async (sections, options, onProgress) => {
    // ...
};

const estimate: typeof estimatePDFSize = (sections, backgrounds) => {
    // ...
};
```

`generate` must honor the third argument - `onProgress(value: number)` reports 0 → 1 to the bundled `GenerationProgress` overlay. `options.signal` is an `AbortSignal` you should respect at every awaitable step.

## Full example - pdf-lib + Worker

```tsx
import type { generatePDF as GenerateFn } from '@docmosaic/core';

const generate: GenerateFn = async (sections, options, onProgress) => {
    const worker = new Worker(new URL('./pdf-worker.ts', import.meta.url), { type: 'module' });
    try {
        return await new Promise<Blob>((resolve, reject) => {
            worker.addEventListener('message', (e) => {
                if (e.data.type === 'progress') onProgress?.(e.data.value);
                if (e.data.type === 'done') resolve(e.data.blob);
                if (e.data.type === 'error') reject(new Error(e.data.message));
            });
            worker.postMessage({ sections, options });
            options.signal?.addEventListener('abort', () => {
                worker.terminate();
                reject(new Error('PDF generation cancelled'));
            });
        });
    } finally {
        worker.terminate();
    }
};

<Editor.Root pdf={{ generate }}>{/* ... */}</Editor.Root>;
```

## Estimation

`estimatePDFSize` is called frequently (every time the document changes) so it must be cheap. If your custom backend can't produce a fast estimate, leave `estimate` undefined - `Editor.FileSizeBadge` will fall back to the bundled heuristic.

## See also

-   [Editor.Root](/docs/primitives/root)
-   [DownloadButton](/docs/primitives/download-button)
-   [usePdfGeneration hook](/docs/reference/hooks)