Every file converter looks the same: a drop zone, a button, a download. The differences that matter — where your file goes, who can read it, how long it exists — are invisible unless you know where to look.
The three processing models
Server-side converters upload your file, process it remotely, and delete it “after a few hours” (their words, unverifiable). Ad-funded free-for-all sites do the same with less accountability and more trackers. Local, browser-based tools — like everything on this site — run the conversion on your own device; the file never transmits at all.
How to verify a converter yourself
- Open your browser’s developer tools → Network tab before converting.
- Run a conversion with a test file and watch the requests.
- A local tool shows no upload of your file’s bytes; a server tool shows a large POST request. Try it here with the PDF compressor.
When server-side is disqualifying
Contracts, medical records, financial statements, ID documents, unreleased business material — anything you would not email to a stranger should not upload to one either. The privacy guide goes deeper, and the offline conversion guide shows the strongest version of the local model. Free without uploads also means free without watermark ransom.