How free online tools make money — and what it means for your privacy

Most free utilities show ads. Ad networks usually set cookies to measure views and, unless you opt out, to personalise ads based on browsing on other sites. That is about your visits, not your files, but it is still data. Reputable sites say so in their privacy policy and let you opt out.

Many tools give away the basics and charge for more: larger files, batch processing, no ads. This model has the cleanest incentives, because the product itself is what's being sold. Watch for free trials that silently turn into subscriptions.

Some services keep uploaded files for a period, or reserve the right to use them to "improve the service" — which can include training machine-learning models. Read the terms for how long files are stored, whether humans can review them and whether they are shared with partners.

A short retention period (for example deletion after one or two hours) is a good sign. Vague wording, or no mention of retention at all, is not.

The worst model bundles adware into downloaded installers, asks for unnecessary browser-extension permissions or sells contact details. Prefer tools that run in the browser without installation, and be suspicious of any converter that insists on an extension or app for a simple job.

You don't need to read every word. Search the page (Ctrl+F) for "upload", "retain" or "delete", "third part", "train" and "sell". Those five searches usually reveal how files are handled, how long they are kept, who else receives data, whether content is used for machine learning and whether data is sold.

Also look at the date. A policy that hasn't changed in many years may no longer describe what the site actually does.

Promises are only as good as the incentives behind them. A site paid by subscriptions has a reason to protect your files, because trust is what it sells. A site that earns from data has a reason to keep as much as it can. Understanding which model a tool uses tells you which promises are likely to hold when nobody is checking.

Does it upload my file? How long is it kept? Who can access it? How is the site paid for? If a site answers all four clearly, you can make an informed choice. If it answers none, assume the least favourable answer.