You can always do the most cherished "Windows" thing....format C:/ and reinstall W7 wich in my opinion runs better than any "8" version,and takes less resources. No less than a few years ago still 70% of Windows users were still with XP,that says a lot about Microsoft OS's quality.So far 7 is the best all around performing OS.
Why are you trying to download DX9 when the Dev's already provide you with the complete DX9 package in the WOFF Toolbox ???
All you have to do is execute it to install the complete DX9 package on to your PC. The normal DX9 web version that you find on the internet is incomplete versus the provided DX9 version in the WOFF Toolbox.
CPU = i9 11900K GPU = RTX 3080 Ti Monitor = ASUS ROG Swift PG32UQX 2160p G-sync
Of course WOFF works on Windows 8 there's a few million people using it I guess, and we'd have posted a message about it not working by now -if it were so!
If you mean you are trying to make CFS3 itself run (not WOFF) then install DX9 anyway and try.
If you mean you can't fly in WOFF then see the FAQ section 40 anyway. DX9 install as per the detailed FAQ, and sticky post in this very forum - it tells you STEP BY STEP how to install DX9 and WHICH DX9 to use. And as Panama Red says we even provide it for you to make it really easy.