\documentclass{article}
\usepackage{amsfonts, amsthm}
\usepackage{hyperref}

\newtheorem{theorem}{Theorem}
\newtheorem{custominner}{Theorem}
\newenvironment{custom}[1]{%
  \renewcommand\thecustominner{#1}%
  \custominner
}{\endcustominner}

\begin{document}

\begin{custom}{1.29}

For every real $x$ there exists a positive integer $n$ such that $n > x$.

\end{custom}

\begin{proof}

\href{Chapter_I_3_10.lean}{Apostol.Chapter\_I\_3\_10.Real.exists\_pnat\_geq\_self}

\end{proof}

\begin{custom}{1.30}[Archimedean Property of the Reals]

If $x > 0$ and if $y$ is an arbitrary real number, there exists a positive integer $n$ such that $nx > y$.

\end{custom}

\begin{proof}

\href{Chapter_I_3_10.lean}{Apostol.Chapter\_I\_3\_10.Real.pos\_imp\_exists\_pnat\_mul\_self\_geq}

\end{proof}

\end{document}