\documentclass[a4paper]{article}
\usepackage[top=0cm, bottom=0cm, left=0cm, right=0cm]{geometry}
\usepackage{graphicx}
\usepackage{tikz}
\begin{document}
\pagestyle{empty}
\begin{tikzpicture}[inner sep=0pt, outer sep=0pt]
\draw node[above right] {\includegraphics[page=1, width=\linewidth]{inp.pdf}};
\begin{scope}[overlay]
\draw (0, 0) grid (20, 30);
\end{scope}
\end{tikzpicture}
\end{document}