Insert pdf, add margin

Question

I'd like to add to a pdf a margin of 1cm. Around this page I will add a frame later and like to draw something else to the frame. However, how can I add a margin to a pdf without scaling the pdf (-> single page document: the old pdf unscaled plus a 1 cm border)?

This is what I have tried it so far:

\documentclass{scrartcl}
\usepackage{pdfpages}
\usepackage{geometry}

\begin{document}
\includepdf[frame, fitpaper]{incl.pdf}
% I thought I'd use fitpaper and add a margin...
\newgeometry{left=3cm,right=1cm} % This does not do anything at all.
\end{document}

Answer 1

The following minimal example creates lipsum.pdf - a single-page document with an image (no border included):

\documentclass{article}
\usepackage{graphicx,lipsum}
\begin{document}
\noindent
\includegraphics[width=\linewidth]{example-image}

\bigskip

\lipsum[1-2]
\end{document}

The following minimal example insert the single-page PDF lipsum.pdf with a 1cm padded border:

\documentclass{article}
\usepackage{pdfpages}
\usepackage{geometry}
% Grab external image for processing
\newsavebox{\externalimage}
\savebox{\externalimage}{\includegraphics{lipsum.pdf}}
\setlength{\fboxsep}{1cm}
\geometry{
  paperwidth=\dimexpr\wd\externalimage+2\fboxrule+2\fboxsep,
  paperheight=\dimexpr\ht\externalimage+2\fboxrule+2\fboxsep,
  margin=0pt
}
\setlength{\parindent}{0pt}
\begin{document}
\fbox{%
  \setlength{\fboxsep}{-\fboxrule}% (remove)
  \fbox{% (remove)
    \usebox{\externalimage}%
  }% (remove)
}
\end{document}

The included PDF remains unscaled as the page geometry is made to fix exactly lipsum.pdf + two rules (left/right or top/bottom, resulting from \fboxrule) + two 1cm gaps (\fboxsep).

For visual effect, I've added a frame around the included page, by that can be (remove)d.