\documentclass[bigger]{beamer}
\usepackage{pdfcomment}
\begin{document}
\begin{frame}
  this is page 1
  \pdfcomment{and an annotation for page 1}
\end{frame}
\begin{frame}
  this is page 2
  \pdfcomment{and another annotation}
\end{frame}
\end{document}
