The official version is —it requires a purchase (around €50 / $60 USD) for a license.
When you see "download SAI" on DA, you are likely encountering one of three things:
Historically, SAI had a 31-day trial. Currently, the developer recommends buying it directly. You can find the official demo/installer at the developer’s site: systemax.jp/en/sai/ (use browser translation if needed).
