There is a hidden cost to "free" online conversion sites, because your file becomes someone else's file the moment you upload it.
What happens when you upload a file
Most online tools push your file to a server, process it there, and let you download the result. Many of those servers cache uploads. Some scan them. Some keep copies for "training". You usually find out by reading the terms of service that nobody reads.
What "browser-based" actually means
A browser-based tool downloads JavaScript code to your computer and runs the processing in that code, on your machine. Nothing leaves your device. You can verify this yourself. Open your browser's developer tools, go to the Network tab, run the tool, and watch for upload requests. There will not be any.
Trade-offs to know about
- The first visit downloads more code than a server-side tool would. The next visit is cached.
- Very large files are limited by your device memory rather than the server's.
- Some specialist formats may need extra libraries that load when you open the tool.
For everything else, browser-based wins on privacy, speed and cost.