\documentclass{article}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[a4paper, margin=0.25cm, landscape]{geometry}


\usepackage{multicol}                       % 2-col formatting
\usepackage{hyperref}
\usepackage[english]{babel}                 % theorem environ
\usepackage{framed}
\usepackage{mathtools}
\usepackage{stmaryrd}                       % more arrows
\usepackage{mathpartir}
\usepackage{graphicx}
\usepackage{tabularx}
\usepackage{ebproof}
\usepackage{amsmath}
\usepackage{amssymb}

\usepackage{subfiles}                       % multi-file project

% environment `definition` prints as "Definition" and has counter reset per section.
\newtheorem{definition}{Definition}[section]
\newtheorem{background}{Background}[section]
\newtheorem{example}{Example}[section]

\begin{document}
\begin{multicols*}{3}

\section{LTS; ACP}
\subfile{01-spec-and-impl}

\section{Semantic Equivalences}
\subfile{02-semantic-equivalences}

\section{CCS; SOS}
\subfile{03-ccs.tex}

\section{Equational Axiomisation}
\subfile{04-congruence.tex}

\end{multicols*}
\end{document}