MendPDF edits PDF documents entirely inside your browser. The file goes from your device into browser memory, gets modified by a local WebAssembly engine, and the result is saved back to your device. No server is involved at any point during editing.
What happens when you edit a PDF in MendPDF
File stays on your device: When you drop a PDF into the editor, it loads into your browser's memory. It is not uploaded anywhere.
WASM engine does the work: A compiled PDFium-based WebAssembly engine runs inside your browser tab. It reads the PDF structure, lets you modify text, and patches the content stream directly.
Save outputs locally: When you save, the modified PDF is generated in browser memory and downloaded to your device. No server round-trip.
Close tab to clear: When you close the browser tab, the file data in memory is discarded. Nothing persists on any server.
Why no-upload editing matters
Every time you upload a document to an online tool, you are trusting that service with your file content, even if only temporarily. For contracts, invoices, personal records, legal documents, or anything you would not send to a stranger by email, a no-upload editor removes that trust requirement entirely. The file never leaves your control.
What you can do in the editor
Text editing: Click on any text to modify it. The engine patches the original PDF structure, preserving fonts and layout.
Font and color controls: Adjust bold, font size, and text color. The editor auto-detects the original properties.
RTL languages: Full support for Arabic and Hebrew with correct bidirectional handling and script-aware fonts.
Annotations: Add text boxes, freehand drawing, shapes, and images as overlay annotations.
Incremental save: The engine patches the PDF content stream directly rather than rewriting the entire file, preserving document integrity.
Browser requirements
MendPDF works in Chrome 57+, Firefox 52+, Safari 11+, and Edge 16+. All modern browsers support the WebAssembly technology that powers the local editing engine. No browser extension or plugin is needed.