I created a LaTeX document using Overleaf and a diagram I made in Numbers and exported in PDF format. I included it into Latex like so:
\documentclass[12pt,a4paper]{article}
\usepackage{fullpage}
\usepackage[bottom]{footmisc}
\usepackage{graphicx}
\usepackage{gensymb}
\usepackage{float}
\usepackage{caption}
\begin{document}
\begin{figure}[h!]
\begin{center}
\includegraphics[width=15cm, trim=1.8cm 11.9cm 2.5cm 1.7cm, clip]{Diagram.pdf}
\end{center}
\end{figure}
\end{document}
And it is being displayed correctly by overleaf:

However, when exporting it as a PDF on Overleaf and printing it, I get this:

This is a link to the PDF used: https://www.dropbox.com/s/kyku3gxe6b9utj8/Gegevens.pdf?dl=0
How do I get these lines to work?

\centeringinstead of\begin{center}...\end{center}. http://tex.stackexchange.com/q/2651/32374 Welcome to TeX.SX! – darthbith Feb 08 '15 at 16:18\documentclass{...}, the required\usepackage's,\begin{document}, and\end{document}. That may seem tedious to you, but think of the extra work it represents for TeX.SX users willing to give you a hand. Help them help you: remove that one hurdle between you and a solution to your problem. – Martin Schröder Feb 09 '15 at 12:34pdfTeX, Version 3.14159265-2.6-1.40.15 (TeX Live 2014). – Martin Schröder Feb 09 '15 at 22:37