You need to merge two PDFs, shrink a scan or sign a form. A quick search shows dozens of free websites that do it in seconds. For a menu or a flyer, any of them is fine. But what about a bank statement, a contract or a copy of your ID?

This guide explains in plain language what happens to your file when you use an online PDF tool, what can go wrong, and how to check any tool yourself in a couple of minutes, without having to trust anyone's marketing, including ours.

Two ways an online PDF tool can work

Upload tools: your file travels over the internet to the company's server. The server does the work and sends the result back. This is how most online converters have worked for years, because heavy processing used to need a powerful machine.

Browser tools: the page loads code into your browser and your own device does the work. The file is read from your disk into your browser's memory and the result is created there.…