Hey, here is the link to download the SDK, you need to first register for the NVIDIA Registered Developer Program (RDP). Then you can download the tools after confirmation.
You will want to download the PhysX: APEX SDK and there are a few versions, so try them all if you wish…
Also more information on the SDK found here.
https://developer.nvidia.com/apex-download-page
Also just so you know, this isn’t for blender, its a standalone binary that allows you to add physics to your meshes. You can then load the apx file into the unreal engine.