An example of generating PDF/A and PDF/X documents using the pdfx package on Overleaf.
\documentclass[11pt]{article}
\usepackage[in]{fullpage}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{upquote}
\usepackage{listings}
\lstset{basicstyle=\ttfamily,columns=fullflexible,xleftmargin=4em}
\usepackage[a-1b]{pdfx} % for PDF/A-1b
% \usepackage[x-1a]{pdfx} % for PDF/X-1a
\title{Creating PDF/A and PDF/X Files}
\author{Overleaf}
\date{}
\begin{document}
\maketitle
Please note that pdfx doesn't make A-1a, A-2a and A-3a standards compliant pdf's since all the features of "tagged" pdf are not yet implemented. Only A-1b, A-2b, A-2u, A-3b and A-3u can be generated. (Thanks to Radhakrishnan CV for pointing this out!)
\begin{enumerate}
\item Load the \texttt{pdfx} package, specifying whether you would like to create a \texttt{a-1b} (PDF/A) or \texttt{x-1a} (PDF/X) document.
\item The timezone may need to be set explicitly. On Overleaf, this can be done in the latexmkrc file:
\begin{lstlisting}
$ENV{'TZ'}='Europe/London';
\end{lstlisting}
or
\begin{lstlisting}
$ENV{'TZ'}='GMT-6';
\end{lstlisting}
A list of supported time zones is available at \url{http://php.net/manual/en/timezones.php}.
\item \textbf{(Optional)} You can provide metadata in \verb|output.xmpdata|, e.g.
\begin{lstlisting}
\Title{Creating PDF/A and PDF/X Files}
\Keywords{pdfTeX\sep PDF/X-1a\sep PDF/A-b}
\Author{LianTze Lim}
\Org{Overleaf}
\end{lstlisting}
Note that this file \emph{must} be named \verb|output.xmpdate| on Overleaf. If it is present, \texttt{pdfx} requires it to contain at least \verb|\Title| and \verb|\Author|. For more information about other kinds of metadata than can be included, see the \texttt{pdfx} documentation at \url{http://texdoc.net/pkg/pdfx}.
\end{enumerate}
\end{document}