What Does "Processed Locally in Your Browser" Mean?
It means the code that resizes or compresses your file runs inside your browser, on your device. The file is read into memory there, not sent to a server.
How it works
When you pick a file on a web page, the browser can hand its contents to the page’s JavaScript without sending it anywhere. The page can then decode it, change it (resize, compress, convert) and offer the result back as a download, all in your device’s memory. No upload step is needed, because the program that does the work was already sent to you with the page.
How to check it yourself
- Open your browser’s developer tools (usually F12) and go to the Network tab.
- Use a tool with a file. For a tool that processes locally, you’ll see only requests for the page and its assets, not a request carrying your file.
- Or go offline: load the page, switch off your internet, and run the tool. If it still works, the processing is on your device. Some features need a one-time download (see below), so use them once while online.
What WillItUpload does
- The site is served as static files. It has no server code that accepts file uploads.
- In testing across image, HEIC, PDF and requirement flows, we saw no requests to other sites and no requests other than ordinary page and asset fetches. Offline use worked for an image tool, a PDF tool and a signature tool.
- Two features download a program once, from this site: the decoder for iPhone HEIC photos, and the reader that pulls requirement text out of a screenshot. That is code, not your file, and the file is still processed on your device.
- When you send a finished image from one tool to the next (for example into the requirement checker), it’s held briefly in your own browser’s storage and deleted as soon as the next page takes it, or after 10 minutes.
- Analytics are anonymous event counts such as “a tool was opened”. They carry no file contents, file names, or text you type into requirement boxes.
What local processing doesn’t cover
It stops your file being sent to a server. It doesn’t protect against malware on your device, a browser extension that can read pages, or someone else using your computer. The file you download lands in your downloads folder as usual.
It also still relies on trust in the site’s code: your browser runs the program the page delivers, and the Network check above is how you confirm, for your own session, that the program isn’t sending your file anywhere.
It also says nothing about a tool’s output quality or whether a portal will accept it. Those are separate.
For the practical comparison with tools that do upload, see are online PDF compressors safe.