MendPDF processes PDF documents locally in your browser. This page explains what that means technically, why it matters for privacy, and what the limitations are.
What is WebAssembly (WASM)?
WebAssembly is a technology supported by all modern browsers (Chrome, Firefox, Safari, Edge) since 2017. It allows compiled code to run inside a browser tab at near-native speed. MendPDF uses a WebAssembly build of PDFium, the same PDF engine used in Chromium, to read and modify PDF documents directly in your browser.
Step by step: what happens when you edit a PDF
1. You open the editor: Your browser loads the MendPDF page and downloads the WASM engine (about 2MB). This is the only network request related to the editor itself.
2. You drop a PDF file: The file is read from your device into the browser's memory (RAM) using the standard File API. No upload occurs.
3. The WASM engine processes it: The engine parses the PDF structure, renders pages for display, and exposes text positions for editing. All of this happens in the browser tab.
4. You make edits: When you click and modify text, the engine patches the PDF content stream directly. It updates the internal PDF objects without rewriting the entire file.
5. You save: The engine serializes the modified PDF into a byte array in memory. Your browser downloads it as a file to your device.
6. You close the tab: The browser releases the memory. The file data is no longer accessible. Nothing was stored on any server.
What about other tools?
The same local processing applies to PDF-to-Word conversion, merge, split, compress, rotate, page numbers, watermark, protect, and unlock. Each tool uses either the WASM engine or JavaScript libraries running in the browser to process the file without server communication.
What are the limitations?
Large files: Very large PDFs (over 100MB) may be slow or run into browser memory limits. Server-based tools can handle larger files because they have more memory available.
OCR: Optical character recognition for scanned documents typically requires server-side processing. MendPDF does not currently offer OCR.
Collaboration: Real-time co-editing requires a shared server. Local processing is single-user by design.
Font availability: The editor includes self-hosted fonts for Latin, Arabic, and Hebrew scripts. Fonts not included in the bundle may render with substitutions.
How to verify it yourself
Open your browser's developer tools (F12), go to the Network tab, then edit a PDF in MendPDF. You will see that no document data is transmitted after the initial page load. The only network activity is loading the editor assets. Your PDF content stays entirely in the browser.