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noto-pdf-ts

Introduction

noto-pdf-ts is a lightweight PDF to image conversion library for Node.js.

Features

  • Simple API - Just a few functions to convert PDF pages to images
  • Memory Efficient - Processes pages one at a time using async generators
  • CJK Support - Full support for Chinese, Japanese, and Korean fonts via PDFium
  • TypeScript - Written in TypeScript with full type definitions
  • Modern - Supports ES2024 await using for automatic resource cleanup
  • Zero Native Dependencies - Uses PDFium WebAssembly for rendering

Understanding PDF Fonts

Font Formats

PDFs support several font formats:

FormatDescription
TrueType (.ttf)Common format by Apple/Microsoft, widely used
OpenType (.otf/.ttf)Extended TrueType with advanced typography features
Type 1Adobe’s legacy PostScript format
CID fontsDesigned for large character sets (CJK)

Embedded vs. Referenced Fonts

PDFs handle fonts in two fundamentally different ways:

Embedded fonts store the actual font data inside the PDF file. This guarantees consistent rendering on any system, but increases file size.

Referenced fonts only store the font name, relying on the viewing system to supply the font. This reduces file size but may cause display problems if the font isn’t available.

The “Tofu” Problem

When a referenced font isn’t available and no suitable substitute exists, text may render as empty rectangles: □□□

These rectangles are called “tofu” (because they look like blocks of tofu). This is particularly common with CJK (Chinese, Japanese, Korean) characters, which require specialized fonts that may not be installed on all systems.

How noto-pdf-ts Handles Fonts

noto-pdf-ts takes a robust approach to font handling:

  1. PDFium-based Rendering - Uses Google’s PDFium library (the same engine behind Chrome’s PDF viewer) compiled to WebAssembly for reliable, high-quality rendering
  2. Bundled Noto CJK Fonts - Includes Noto Sans CJK fonts out of the box, eliminating tofu for CJK text even when PDFs lack embedded fonts
  3. Custom Font Registration - Allows registering additional fonts for specialized use cases
  4. Missing Glyph Detection - Reports characters that couldn’t be rendered, helping identify problematic PDFs

This approach ensures consistent rendering across platforms, with excellent CJK support by default.

Installation

Terminal window
npm install noto-pdf-ts

Quick Example

import { openPdf } from 'noto-pdf-ts';
import fs from 'node:fs/promises';


// Open a PDF file
const pdf = await openPdf('/path/to/document.pdf');


// Render all pages
for await (const page of pdf.renderPages()) {
  await fs.writeFile(`page-${page.pageNumber}.jpg`, page.buffer);
}


// Don't forget to close
await pdf.close();

Requirements

  • Node.js 20 or later

Next Steps