I am reading a paper in fractional elliptic. It is true that $(-\Delta)^{-s}$ is continuous from $H^{-s}$ to $H^{s}$. However, in the following argument, I cannot find a good reference for it:

Fractional Paper

Can anyone explain or give me some references about the isomorphism?

Here is the full article: https://www.sciencedirect.com/science/article/pii/S002203961400196X