\documentclass{article}
% This is the main document.
%
% The different document parts are loaded using the 'subfiles' package.


% For compiling subdocuments separately.
\usepackage{subfiles}

% To get automatic line breaks in tables.
\usepackage{tabularx}

% Colors, mainly red
\usepackage{color}

% To include figures
\usepackage{graphicx}

% graphics path
\graphicspath{{img/}{../img/}}

% force figure placment
\usepackage{float}

\begin{document}
\subfile{tex/titlepage}

\subfile{tex/abstract}
\newpage

\subfile{tex/contents}
\newpage

\subfile{tex/introduction}

\subfile{tex/project_plan}

\subfile{tex/preliminary_study}

\subfile{tex/lifecycle_model}

\subfile{tex/requirements}

\subfile{tex/sprint1}

\subfile{tex/sprint2}

\subfile{tex/sprint3}

\subfile{tex/conclusion}

\subfile{tex/appendices}

\end{document}

\documentclass{article}
% This is the main document.
%
% The different document parts are loaded using the 'subfiles' package.


% For compiling subdocuments separately.
\usepackage{subfiles}

% To get automatic line breaks in tables.
\usepackage{tabularx}

% Colors, mainly red
\usepackage{color}

% To include figures
\usepackage{graphicx}

% graphics path
\graphicspath{{img/}{../img/}}

% force figure placment
\usepackage{float}

\begin{document}
\renewcommand{\thepage}{\roman{page}}
\subfile{tex/titlepage}

\subfile{tex/abstract}
\newpage

\subfile{tex/contents}
\newpage

\setcounter{page}{1}
\renewcommand{\thepage}{\arabic{page}}
\subfile{tex/introduction}

\subfile{tex/project_plan}

\subfile{tex/preliminary_study}

\subfile{tex/lifecycle_model}

\subfile{tex/requirements}

\subfile{tex/sprint1}

\subfile{tex/sprint2}

\subfile{tex/sprint3}

\subfile{tex/conclusion}

\subfile{tex/appendices}

\end{document}

Make initial pages use roman numerals.
Make initial pages use roman numerals.
