Are my PDF files uploaded?

No. The document is opened and rewritten inside your browser. It is never sent to a server, and closing the tab discards it.

Does it work on scanned documents?

Use the visual view. The text view needs a text layer, which scans do not have unless OCR has been run.

Pages are compared in order, so an inserted page makes every later page look different. Use Organize PDF to line the pages up first.

JavaScript is turned off. InspireTools processes files in your browser, so the tools need JavaScript to run.

Find every difference between two versions of a PDF, in the text and on the page.