Today CTAN Announcements informed that the package blowup has been released. It can be used to upscale or downscale all pages of a document. For a first test I created this short a5paper document:

\documentclass[pdftex,a5paper,10pt]{article}
\usepackage[english]{babel}
\usepackage{blindtext}
\usepackage{hyperref}
\begin{document}
\tableofcontents
 
\section{One}
\blindtext[3]
 
\section{Two}
\blindtext[4]
 
\end{document}

It’s pdf output can be opened here. The pdfinfo tool printed:
Page size: 419.528 x 595.276 pts

Now I used the \blowUp macro to upscale the document pages to fit a4 paper size:

\documentclass[a5paper,10pt]{article}
\usepackage[english]{babel}
\usepackage{blindtext}
\usepackage{hyperref}
\usepackage{blowup}
\blowUp{paper=a4}
\begin{document}
\tableofcontents
 
\section{One}
\blindtext[3]
 
\section{Two}
\blindtext[4]
 
\end{document}

The new pdf output can be opened here. pdfinfo output:
Page size: 595.276 x 841.89 pts (A4)

Of couse you could inspect the paper sizes by using the Adobe Reader.

The blowup package includes several source code examples. During the test I’ve noticed one small mistake in the documentation, the user-level macro \blowUp is described as \blowup, that will cause an error. But I’m sure this will be corrected soon, at least I will report it to the package author.