I don't see any mention of \setlist[enumerate]{font=\bfseries} in the documentation of the paralist package. It's rather part of the enumitem package.
However, playing with some paralist environments could be illuminating and get you somewhere close to what you want:
\documentclass{article}
\usepackage{paralist}
\setdefaultenum{\bfseries 1.}
{\bfseries (a)}
{\bfseries i.}
{\bfseries A.}
\begin{document}
\section*{Customizing \texttt{paralist} enumerations}
\noindent
To customize enumerated \verb|inparaenum| lists with the \verb|paralist| package, we can set bold numbers through \verb|\setdefaultenum{\bfseries 1.}{}{}{}|, where lists will look like:
%
\begin{inparaenum}
\item first item
\item second item
\item third item
\end{inparaenum}\\[1ex]
%
\ldots and items with nested \verb|enumerate|s will look like:
%
\begin{enumerate}
\item text 1
\begin{enumerate}
\item text 1.1
\begin{enumerate}[\bfseries $<$ i $>$]
\item text 1.1.1
\item text 1.1.2
\end{enumerate}
\item text 1.2
\begin{enumerate}[$<$ i $>$]
\item text 1.2.1
\item[$>$ \emph{ii} $<$] text 1.2.2
\end{enumerate}
\item text 1.3
\end{enumerate}
\item[\labelenumi] Note that setting a custom item label with \verb|\labelenumi| does not increase the item numbering.
\item \verb|\theenumi| gives ``\theenumi'' whereas \verb|\labelenumi| gives ``{\labelenumi}'' (in bold plus a dot).
\end{enumerate}
\end{document}
producing items which have bold numbers till it isn't re-customized at the list- or item-levels.

paralistandenumitem? 2) Is there any global way like\setlist[enumerate]{font=\bfseries}in my code? 3) Your solution doesn't work with Greek numbering (α', β', γ' δ',...). How can I fix it? – Kώστας Κούδας Feb 13 '19 at 10:22