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How In-Browser File Processing Works (and Why It's More Private)

By Antuparthi Manoha Malik PaulPublished 3 October 20265 min read

What it means when a tool “runs in your browser”, how that keeps files private, and when a server is genuinely needed.

Expensico tools say “Runs in your browser — your files are never uploaded.” Here's what that means technically, why it's more private, and which jobs genuinely need a server.

Traditional online converters

Most online converters upload your file to their server, convert it there and send the result back. Your document — a bank statement, an ID card, a contract — sits on someone else's computer, at least for a while, and you have to trust what they do with it.

How in-browser processing works

Modern browsers can run substantial software. When you open a tool page, your browser downloads the tool's code — JavaScript and sometimes WebAssembly — and runs it on your own device. When you choose a file, the browser hands its contents to that code directly from your disk. The conversion happens in your device's memory, and the result is offered back to you as a download.

Some of the open-source engines involved:

  • PDF.js (from Mozilla) to read and render PDF pages.
  • pdf-lib to merge, split and rotate PDFs.
  • SheetJS to read and write Excel files.
  • Mammoth to convert Word documents to clean HTML.
  • The browser's own image decoders and encoders for JPG, PNG and WebP.

How you can check

You can verify this yourself. Open your browser's developer tools (F12), switch to the Network tab and run a conversion: you'll see the tool's code being loaded, but your file isn't sent anywhere. After a page has loaded, many tools even keep working with your internet connection switched off.

Jobs that genuinely need a server

A few jobs need software that can't practically run in a browser. Converting a Word, Excel or PowerPoint file to PDF with its exact layout requires a full office suite such as LibreOffice. Rather than offer a lower-quality browser version, Expensico doesn't provide those conversions at the moment — every tool on the site runs in your browser. If we add them, they will run on a dedicated server, be clearly marked as uploading files, and delete your file as soon as the conversion finishes.

Limits of in-browser processing

  • Memory: very large files are limited by your device's memory, especially on phones.
  • Speed: processing uses your device's processor, so older phones may be slower.
  • Formats: some formats, such as iPhone HEIC photos, aren't supported by most browsers.

For most everyday tasks — PDF to JPG, compressing images, merging PDFs, viewing spreadsheets — your browser is more than capable, and your files never have to leave your hands.