It is \usepackage[light,math]{iwona}
\documentclass{article}
\usepackage{amsmath}
\usepackage[light,math]{iwona}
\begin{document}
% bottom of page 2 in the linked document
In addition, we have the following assumptions:
\begin{itemize}
\item $X_{t-1}\in\mathbf{B}(X)$ and $X_{t-1}\cap X_{t'-1}=\emptyset$, $\forall t\ne t'$.
\item $A_t\in\mathbf{B}(A)$ and $A_{t-1}\cap A_{t'-1}=\emptyset$, $\forall t\ne t'$.
\item $j(A_t)=X_{t-1}$ or $j^{-1}(X_{t-1})=A_t$ and specifies the admissible control
space for state $X_{t-1}$, $\forall t$.
\end{itemize}
\end{document}

If you need the norm symbol \| (as well as \lVert or \rVert), see my answer to How can I use \lVert and \rVert norm symbols (‖x‖) with the Iwona math font?
\usepackage{iwona}Caveat: if you need the norm symbol, see https://tex.stackexchange.com/a/114834/4427 – egreg Feb 03 '21 at 21:45