The download can do most things but it cannot save unless you buy it.
WriteItNow runs on PCs (Windows 7, 8 and 10) and Macs (OS X)
if the free demo runs on your computer the full version will as well.
WriteItNow 6 has a lot more features than WriteItNow 4. If you bought 4 you can buy an upgrade to 5 or 6