The archive format both Windows and macOS open by double-clicking, which is most of why it won. Each file inside is compressed on its own, so you can pull one out without unpacking the rest - handy for browsing, slightly worse for compression ratio.

Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a ZIP file above and choose afterwards.