draw on pdf file

Question

I have such a case: I got one pdf file but it's read only,I wish to draw on it. so I think maybe I can do it as below: 1). embed origin.pdf into latex code as a picture. 2). find right position to draw on it.

But I wish the new pdf file dimension exactly the same as origin one.

I try below code, but it seems the original pdf file (8.5inx11.0in) has been cropped.

\documentclass{article}
\usepackage[paperwidth=8.50in, paperheight=11.00in]{geometry}
\usepackage{tikz}

\begin{document}

\tikzstyle{default}=[anchor=north west,draw=red,inner sep=0pt]
\begin{tikzpicture}[anchor=south west,inner sep=0pt, outer sep=0pt]
\node at (current page.south west) {
    \begin{tikzpicture}[overlay,remember picture,inner sep=0pt, outer sep=0pt,anchor=south west]
        \node[anchor=south west,inner sep=0,rotate=0] (image) at (0,0) {\includegraphics{origin.pdf}};
    \end{tikzpicture}
};

\end{tikzpicture}

\end{document}

Not sure if it's the right one to do such tasks!