Error with using \pdfpxdimen: "Undefined control sequence"

Question

I am trying to change the DPI of the output PDF using \pdfpxdimen in the preamble, but it returns an error:

"! Undefined control sequence \pdfpxdimen".

I guess that is because I am using XeLaTeX instead of pdfLaTeX. If it is so, is there any way to change the DPI using XeLaTeX?

Here is the MWE:

\documentclass[a4paper,11pt]{article}

\pdfpxdimen=1in % 1 DPI
\divide\pdfpxdimen by 96 % 96 DPI

\begin{document}
some text
\end{document}

Answer 1

Your code doesn't "change the dpi". You are only changing the value of the "px" unit. Try this with pdflatex to understand what's happening:

\documentclass[a4paper,12pt]{report}
\begin{document}

\pdfpxdimen=1in % 
a\hspace{1px}b 

\pdfpxdimen=2in % 
a\hspace{1px}b
\end{document}