The short version
Scissor PDF never receives your files. When you open a tool, your browser downloads the website and a PDF engine. When you choose a file, it is read into the memory of your browser tab, processed by that engine on your own processor, and the result is handed back to you as a download. Closing the tab erases everything.
What runs where
The PDF engine that merges, splits, reorders, rotates, compresses, numbers, watermarks, encrypts and decrypts PDFs is written in Rust, using the open-source lopdf library, and compiled to WebAssembly. WebAssembly is a sandboxed format every modern browser runs at close to native speed. It runs in a background worker so the page stays responsive.
Page previews and PDF to JPG use PDF.js, the open-source renderer built into Firefox, also running locally.
The server is a plain static file host. It has no upload endpoint, no database and no application code, so there is nothing that could accept a document.
Verify it yourself
Open your browser's developer tools, switch to the Network tab and use any tool. You will see the page's own files load, and no request carrying your document. You can also disconnect from the internet after the page has loaded: every tool keeps working.
The site also sends a strict Content Security Policy that tells your browser to block the page from contacting any other server, so even a mistake in our code could not send your files elsewhere.
Works offline, installs like an app
After your first visit, a service worker stores the site and engine in your browser's cache, so the tools open and work with no connection. In Chrome or Edge, choose Install from the address bar. In Safari, choose Add to Dock (Mac) or Add to Home Screen (iPhone and iPad).
Limits of local processing
Because the work happens on your device, very large files are limited by its memory. Several hundred megabytes are fine on a typical laptop, and phones handle everyday documents. Speed depends on your processor rather than on a server queue, which in practice is usually faster than uploading and downloading.