Local vs upload-based file tools

Almost every free file converter uploads your document to a server. A browser can do the same work without sending anything. Here is the actual difference, including where the upload approach still wins.

The comparison

Runs in your browser Uploads to a server
Who sees your file Only you The operator, plus anyone with access to their storage
Time cost Processing only Upload + queue + processing + download
File size limit Your device’s memory Whatever keeps their bandwidth bill down
Works offline Yes, once the page has loaded No
Account required No Often, for anything beyond the basics
Heavy operations Limited by your hardware Can use far more memory and CPU than a phone has
Exotic formats Only what browsers support Can run any software they like

Where upload-based tools genuinely win

Two cases, and they are real rather than concessions for the sake of appearing balanced.

The first is heavy work on weak hardware. A server with 32 GB of RAM can restructure a 2 GB PDF that would exhaust a phone. Browser tabs have a memory ceiling, and a large enough job will hit it.

The second is unusual formats. A server can run a full office suite or a decades old codec. A browser is limited to what it and WebAssembly can do, which covers the common cases well and the obscure ones not at all. Real PDF compression is one such gap here, because every high-quality engine that does it is licensed AGPL.

How to tell which kind you are using

You do not have to trust anyone’s claim, including ours. Open your browser’s developer tools with F12, switch to the Network tab, and run the tool. If your file is being uploaded you will see a request whose size matches it. If nothing of that size appears, the work is happening locally.

A quicker heuristic: drop your connection after the page has loaded. A local tool keeps working. An upload-based one cannot.

The tools here

All 52 of these run locally, with no upload, account or watermark:

More detail on how they are built, and what they cannot do, is on the about page. We keep no copies because we never receive any — see the privacy policy.