How WebAssembly Powers Client-Side PDF Processing
Introduction
For decades, processing PDFs on the web meant one thing: uploading your document to a server. Every merge, compression, and conversion required sending potentially sensitive files across the internet. ILikePdf changes this by running all PDF processing entirely in your browser using WebAssembly (WASM). This article explains how we compile Ghostscript and other PDF libraries to WASM, manage memory across large documents, and deliver near-native performance without a single server upload.
The WASM Advantage
WebAssembly is a low-level binary instruction format that runs in the browser at near-native speed. Unlike JavaScript, which is interpreted or JIT-compiled, WASM is compiled ahead of time into a compact binary that maps directly to CPU instructions. This makes it possible to take established C/C++ libraries like Ghostscript, pdf-lib, and libjpeg and run them in the browser with minimal performance overhead.
WASM's sandboxed execution model provides a natural security boundary. Each module runs in its own linear memory space, isolated from the page and other modules. This is critical for PDF processing — PDFs can contain embedded JavaScript, hyperlinks, and external references. By containing the parser in a WASM sandbox, malicious document features cannot escape the processing context.
How Ghostscript Runs In-Browser
Ghostscript is the industry-standard PDF and PostScript interpreter, written in C. To run it in the browser, we compile it to WebAssembly using Emscripten, a toolchain that compiles C/C++ to WASM and provides a JavaScript glue layer for interacting with the browser environment.
The compilation process involves several steps. First, we patch Ghostscript's build system to target Emscripten's LLVM backend instead of native x86 or ARM. We disable threading features that the browser doesn't fully support and replace file I/O calls with Emscripten's virtual filesystem (MEMFS), which keeps all data in RAM. The resulting WASM binary is approximately 8 MB compressed — a one-time download that caches locally via the service worker.
At runtime, when a user uploads a PDF for compression, the file bytes are read as an ArrayBuffer and written into the virtual filesystem. Ghostscript processes the file in-memory, and the output is read back as a typed array, converted to a Blob, and offered for download. The entire pipeline never touches a network socket.
Performance Benchmarks
We benchmarked ILikePdf's WASM-based compression against a traditional server-side Ghostscript setup. The server ran on an AWS EC2 t3.medium instance. The client WASM ran in Chrome 125 on an M2 MacBook Air.
- Small PDF (1 MB, text-only): WASM 0.4s · Server 1.1s (including upload)
- Medium PDF (15 MB, mixed text and images): WASM 2.1s · Server 3.8s
- Large PDF (85 MB, scanned pages): WASM 8.3s · Server 14.2s
- Peak memory usage (85 MB file): WASM 340 MB · Server 420 MB
The key insight: WASM eliminates network transfer time, making it faster even when the raw processing speed is slightly below native. For mobile users on slow connections, the difference is even more dramatic — server-side tools require a full upload before processing even begins.
Memory Management Strategies
WASM's linear memory is a fixed-size array. When processing large PDFs, we must carefully manage this space to avoid out-of-memory errors. ILikePdf uses a multi-tier approach. First, we estimate memory requirements from the file size and page count before loading the WASM module. If the estimated need exceeds available memory, we display a clear error message rather than crashing the tab.
Second, we process pages in batches for operations like PDF-to-image conversion. Instead of rendering all 200 pages at once, we process 20 pages, flush the memory, then proceed. This keeps peak memory usage predictable regardless of document length.
Third, we use SharedArrayBuffer (where available) to share memory between the main thread and a Web Worker, keeping the UI responsive during long operations. The worker handles Ghostscript execution while the main thread updates a progress indicator.
Privacy Benefits
The most important consequence of WASM-based processing is privacy. Since all computation happens in the browser, ILikePdf never receives your documents. There are no upload servers to breach, no logs of processed files, and no third-party APIs handling your data. This zero-knowledge architecture means ILikePdf cannot access, store, or share your documents even if compelled to do so.
For enterprises handling GDPR-protected data, HIPAA-regulated medical records, or attorney-client privileged documents, this eliminates an entire class of risk. You get the convenience of a web tool with the privacy guarantee of offline software.