What free means here
Creating an account costs nothing, and a free account comes with monthly credits. The document you download is the same one a paid plan gets: no watermark, no pages left out, nothing added to your text.
Why not install a converter?
You can. This tool runs on pandoc, which is free and open source. Installing and setting it up is worth it if you convert files every day. For a file now and then, dropping it in the browser is faster.
Opens everywhere Word files open
The output is a standard .docx. It opens in Microsoft Word, Google Docs, LibreOffice and Pages, and headings use the built-in heading styles, so a table of contents can be generated in two clicks.