Now grab your pen, plug in that drive, and start drawing—anywhere, anytime.

Paint Tool SAI is widely considered "portable" because of its minimal system footprint and installation style: Zip Extraction

The jump from the original Paint Tool SAI (Ver 1) to Ver 2 was significant. While the interface remained minimalist, the engine under the hood was overhauled.

If you are upgrading from the original SAI, the portable version of SAI 2 offers several massive improvements:

Unlike the original, which struggled with large canvases, SAI 2 supports massive resolutions and high bit-depths.