How to include existing PDF slides into my Beamer presentation?

To import a slide from another beamer presentation as a "picture", you can use pdfpages. You'll have to \setbeamercolor{background canvas}{bg=} as explained in this answer because only a white frame will be shown otherwise:

\documentclass{beamer}
\usepackage{pdfpages}
\begin{document}
\begin{frame}
Content
\end{frame}
{
\setbeamercolor{background canvas}{bg=}
\includepdf[pages=3]{filea.pdf}
}
\end{document}

---

However, if you use head- or footlines or even a different theme in your existing presentation, this will probably not fit snugly. In this case, a better solution would be to actually import the source code of the existing presentation, which can be done using the docmute package in combination with the \againframe command:

Suppose this is your existing presentation filea.tex: You need to give the slides you want to import a name using the label option of the frame environment, like this:

\documentclass{beamer}
\begin{document}
\begin{frame}[label=myframe]
Frame to be included
\end{frame}
\end{document}

Then you can use this code in your new presentation:

\documentclass{beamer}

\usepackage{docmute}
\makeatletter
\newcommand*{\loadpresentation}[1]{{\beamer@inlecturefalse\input{#1}}}
\makeatother

\begin{document}
\loadpresentation{filea.tex}
\begin{frame}
  The new presentation
\end{frame}
\againframe{myframe}
\end{document}

Issuing \loadpresentation{filea.tex} imports the frames from your existing presentation without displaying them. You can insert them wherever needed using \againframe with the label you chose in filea.tex. The command \loadpresentation should be used someplace after \begin{document} (but before you actually include a frame from this presentation, of course).

This works roughly the same as if you'd actually copy the source code of the frame from the existing presentation, so things like overlays etc. are taken over.

If you want to put a "thumbnail" of some target beamer PDF fileA in your source, you can use

\includegraphics[page=<n>,width=<len>]{fileA}

where <n> is the absolute page number of the slide in fileA.pdf, and <len> is the width of the imported page. You can also scale this using the other options provided by graphicx. beamer loads graphicx by default, so the functionality is already available.

If you want to put a "full page" of some target beamer PDF fileA in your source, you can use

\begin{frame}
\vspace*{-1pt}
\makebox[\linewidth]{\includegraphics[page=<n>,width=\paperwidth]{fileA}}
\end{frame}

where <n> is the absolute page number of the slide in fileA. The slide is stretched to fit \paperwidth so should overlay the source presentation completely.

The vertical correction (1pt) is needed to maintain a consistent display from one slide to the next.