Digitally sign PDF documents client-side using canvas signature drawings.
or click to browse local folders
Supports PDF/Image sizes up to 500MB
Executing operation locally in-browser...
Your processed document is ready for download.
Process Another FileDigitally sign PDF documents client-side using canvas signature drawings. Process your documents 100% client-side in your web browser with zero file uploads, no signups, and no limits.
Run add signature operations in real-time. Built with pdf-lib + Canvas, document manipulation completes locally in browser memory sandbox within milliseconds.
Your confidential documents never touch external cloud servers. All processing happens 100% client-side on your local device for maximum data security.
No subscriptions, no hidden fees, no email registration, and zero watermarks stamped on your final PDF files.
The free online Add Signature tool on PDFThunder.com lets you digitally sign pdf documents client-side using canvas signature drawings. effortlessly. Whether you are working with corporate contracts, legal briefs, tax filings, or academic papers, our browser-based utility delivers professional-grade document formatting without needing expensive software installations like Adobe Acrobat Pro. Because PDFThunder compiles document streams locally using WebAssembly and JavaScript engines like pdf-lib + Canvas, your files are never uploaded across the internet, protecting you from data leaks and network transfer delays.
Files are read and compiled using client-side memory buffers, making add signature up to 10x faster than cloud converters.
Works on all web browsers (Chrome, Safari, Firefox, Edge) across Windows, macOS, Linux, iOS, Android, and Chromebooks.
Popular PDF web converters like iLovePDF, SmallPDF, and Sejda require uploading your sensitive files to cloud servers, which introduces privacy vulnerabilities and imposes strict daily file size limits unless you purchase a paid subscription. PDFThunder is built from the ground up as a 100% free, browser-native alternative. We impose zero file caps, require no user accounts, and never store or see your document contents. You can even load this page and use add signature completely offline without an active internet connection!
Ctrl+D / Cmd+D) for 1-click access whenever you need to digitally sign pdf documents client-side using canvas signature drawings..PDFThunder provides basic self-signed electronic annotations suitable for standard approvals.