Break PDF booklet into pages

Question

I have a PDF with booklet-odered pages (like the one below). I would like to break these PDF A4 pages back into the original twice as many PDF A5 pages.

|-------| PDF page 1
| 8   1 |
|-------|

|-------| PDF page 2
| 2   7 |
|-------|

|-------| PDF page 3
| 6   3 |
|-------|

|-------| PDF page 4
| 4   5 |
|-------|

Answer 1

Here's a solution via tikz and graphicx. Two compilations are required. gen.pdf is your booklet PDF file.

\documentclass[a5paper]{article}
\usepackage{geometry}
\usepackage{tikz}
\pagestyle{empty}
\begin{document}
\foreach \page in {1,...,4}{
  \begin{tikzpicture}[overlay,remember picture]
    \ifodd\page
    \node at (current page)
    {\includegraphics[page=\page,trim=148.5mm 0 0 0]{gen.pdf}};
    \else
    \node at (current page)
    {\includegraphics[page=\page,trim=0 0 148.5mm 0]{gen.pdf}};
    \fi
  \end{tikzpicture}
  \newpage
}
\foreach \page in {4,...,1}{
  \begin{tikzpicture}[overlay,remember picture]
    \ifodd\page
    \node at (current page)
    {\includegraphics[page=\page,trim= 0 0 148.5mm 0]{gen.pdf}};
    \else
    \node at (current page)
    {\includegraphics[page=\page,trim= 148.5mm 0 0 0]{gen.pdf}};
    \fi
  \end{tikzpicture}
  \newpage
}
\end{document}

Answer 2

Modification of the solution by PolGab, I got away the tikzpicture part which is completely unnecessary IMO, and I use tikz only for \foreach.

\documentclass[a5paper]{article}
\usepackage[margin=0pt]{geometry}
\usepackage{tikz}
\pagestyle{empty}
\def\placepage#1{%
\leavevmode\kern-0.25in{#1}%
}
\begin{document}
\foreach \page in {1,...,4}{
    \ifodd\page
    \placepage{\includegraphics[page=\page,trim=148.5mm 0 0 0]{y.pdf}}
    \else
    \placepage{\includegraphics[page=\page,trim=0 0 148.5mm 0]{y.pdf}}
    \fi
  \newpage
}
\foreach \page in {4,...,1}{
    \ifodd\page
    \placepage{\includegraphics[page=\page,trim= 0 0 148.5mm 0]{y.pdf}}
    \else
    \placepage{\includegraphics[page=\page,trim= 148.5mm 0 0 0]{y.pdf}}
    \fi
  \newpage
}
\end{document}

Answer 3

... and here are detailed descriptions how you can do that with the help of Ghostscript:

• Linux-based tool to chop PDFs into multiple pages (Superuser.com)
• Freeware to split a pdf's pages down the middle? (Superuser.com)
• Convert PDF 2 sides per page to 1 side per page (Superuser.com)