Because it is proprietary software, there is no official "free" full download for REFPROP 9.1. You can obtain it through the following official channels:

If you need REFPROP for your work, there are better (and safer) ways to get it than hunting for a sketchy download link:

NIST REFPROP (REference Fluid PROPerties) Version 9.1 is a legacy version of the software developed by the National Institute of Standards and Technology (NIST) to calculate the thermodynamic and transport properties of industrially important fluids and their mixtures. The Evolution of REFPROP 9.1

The official version of NIST REFPROP 9.1 superseded by Version 10.0