earonesty/boxpdf](https:

Stream PDFs page-by-page with bounded memory, regardless of total page count.

New: `streamFlow`

```ts
import { PDFDocument, StandardFonts } from "pdf-lib";
import { streamFlow, text } from "boxpdf";

const pdf = await PDFDocument.create();
const font = await pdf.embedFont(StandardFonts.Helvetica);

// Workers / edge — stream into the Response body.
const { readable, writable } = new TransformStream<Uint8Array, Uint8Array>();
streamFlow(pdf, writable, generateRows(font)).catch(console.error);
return new Response(readable, { headers: { "content-type": "application/pdf" } });

async function* generateRows(font) {
for await (const order of fetchOrders()) {
yield buildRow(font, order); // GC-able after this yield is consumed
}
}
```

For Node, wrap a Writable:

```ts
import { createWriteStream } from "node:fs";
import { streamFlow, nodeAdapter } from "boxpdf";

const out = nodeAdapter(createWriteStream("./report.pdf"));
await streamFlow(pdf, out, nodes);
```

Memory bench (50 lines/page)

| pages | renderFlow Δheap | streamFlow Δheap | output |
| ---: | ---: | ---: | ---: |
| 50 | 6.1 MB | 0 MB | 70 KB |
| 250 | 35.2 MB | 0 MB | 347 KB |
| 1000 | 157.4 MB | 0 MB | 1.4 MB |

A 1000-page report peaks at 0 MB above baseline with streamFlow vs. 157 MB with renderFlow + pdf.save(). Same byte output within 0.2%.

Contract

1. Embed before streaming. All `embedFont` / `embedJpg` calls must complete BEFORE `streamFlow`. Mid-stream embedding throws with a clear message.
2. Lazy input. Pass a generator — don't materialize the whole Node[].
3. Exclusive writable. streamFlow takes ownership; closes on success, aborts on failure.
4. No `totalPages` in headers/footers. Accessing `ctx.totalPages` throws — switch to `renderFlow` if you need "Page X of Y".
5. ~5% size overhead. Per-batch ObjStm packing is slightly less efficient than whole-doc compression.

Internals

• Built on pdf-lib's public `PDFObjectStream` + `PDFCrossRefStream` exports. Zero deep imports, no fork.
• Per-page snapshot/delta of `PDFContext.indirectObjects`. PDFStream objects (content streams, image XObjects) get written + `ctx.delete()`d immediately. PDFDicts (page dicts, annots) batch into compressed ObjStms (`objectsPerStream` default 50).
• Cross-reference stream uses PDF 1.5+ format — keeps output competitive with renderFlow's default `save()`.

Tests + design

• 12 new tests; 113 total passing
• Full design rationale in `docs/design/streaming.md`
• Comparison harness: `pnpm exec tsx scripts/compare-stream.ts`
• Memory bench: `node --expose-gc --import tsx scripts/bench-memory.ts`