How to check that a tool keeps your files on your device

Every tool listed here is a static web page. When you open a file in one, the page reads it inside your browser tab; there is no upload form and no account.

You do not have to take that on trust. Open your browser's developer tools, switch to the Network tab, then load a file into the tool. Requests for the page's own scripts and styles are expected; a request that carries your file is not, and you should not see one.

A second check is to load the tool, disconnect from the network, and keep working. Each tool's own privacy page says what, if anything, it stores in your browser.