I know that the following can't work. I want the PDF to sit at {X}.
\documentclass{article}
\pagestyle{empty}
\usepackage{tikz,lmodern}
\usepackage{pdfpages}
\begin{document}
\includepdf[scale=0.8, pagecommand={
\begin{tikzpicture}[remember picture,overlay]
\node at ([xshift=7cm,yshift=-3cm] current page.north west) {X};
\end{tikzpicture}}]{test}
\end{document}
I'd like to know how can I position and scale the PDF inside a tikzpicture.

\node at (0,0){\includegraphics{Chick1}}since it is the preferred approach; see Tikz: using external images as building blocks. and section 106 Declaring and Using Images pgfmanual v 3.0.0. – Claudio Fiandrino Apr 02 '14 at 05:26