I cannot find it.
It’s not where I would expect it to be: http://docs.nvidia.com/cuda/index.html
Thank you for alerting us to this issue. I have notified relevant parties about this. Sorry for the inconvenience.
The NPP documentation is only available in PDF form in CUDA 5.0. It should be at doc/pdf/NPP_Library.pdf, within your toolkit installation folder.
A link to the CUDA 5.0 NPP documentation (as a PDF) can be found at the bottom of the following page:
Look for the green link titled “NPP User Guide”.