How Browser-Based File Compression Protects Privacy

Upload-based vs browser-based processing

With an upload-based tool you have to trust the provider’s promises about deleting files, securing storage and limiting staff access. With a browser-based tool, there is simply no copy on a server to protect.

How SlimMyFile processes files locally

When you open a SlimMyFile tool, your browser downloads the website’s code, including compression engines compiled to WebAssembly: MozJPEG for JPEG, libwebp for WebP, OxiPNG for PNG, and pdf-lib and PDF.js for PDFs. When you select a file, the browser reads it into memory and passes it to a Web Worker, a background thread on your device, which does the work. The result is created in memory and saved with your browser’s normal download function.

Files are not written to persistent browser storage. Closing or reloading the page discards them.

What still goes over the network

Local processing does not mean nothing is transmitted. Being precise about it:

• Your browser requests the web pages, styles, fonts and compression engines from the hosting provider. Like any website, the host receives technical data such as your IP address and browser type.
• If advertising is enabled on the site, advertising scripts load from the ad provider and may use cookies, subject to your consent where the law requires it. They do not receive your files or file names.
• Your files themselves, and their names, are not sent anywhere by the tools.

The Privacy Policy describes exactly which services are involved.

Check it yourself in two minutes

You do not have to take any website’s word for it. In a desktop browser:

1. Open a tool page, for example the image compressor.
2. Open the developer tools (F12, or Ctrl+Shift+I, or Cmd+Option+I on a Mac) and select the Network tab.
3. Clear the list, then add a file and run the compression.
4. Look at the requests that appear. You may see the compression engine being downloaded the first time, but there should be no upload: no large POST request carrying your file.

The limits of local processing

• Performance depends on your device. Large PDFs or many high-resolution images take longer on a phone than on a computer, and very large files can run out of memory.
• You still trust the code. A browser-based tool runs code delivered by the website, so it should come from a source you consider trustworthy. The network check above lets you confirm its behaviour.
• Your device must be secure. Local processing protects files in transit and on servers; it cannot protect a device that is already compromised.

When privacy matters most

Identity documents, contracts, medical letters, bank statements, payslips and personal photos are exactly the kinds of files people need to shrink for upload portals and email. For these, a tool that never uploads the file removes an entire category of risk. Try the PDF compressor on a document you would rather not hand to a stranger.

Frequently asked questions