Rearrange pdf pages (swap odd pages and even pages)

Question

I have a pdf and I am in need to replace even and odd pages. If I want to be more specific, I must replace (2n-1) page numbers with (2n) ones (1-->2, 2-->1, 3-->4, 4-->3, and the like). How should I do that?

Is there any free software that can do this job for me?

Answer 1

With poppler-utils tools you could extract the pages to separate files, reorder & save them into an array and join the elements of that array to produce a new document:

pdfseparate original.pdf piece-%04d.pdf
reordered=()
set -- piece*.pdf
while (($#)); do
  if [ $# -ge 2 ]; then
    reordered+=("$2" "$1")
    shift 2
  else
    reordered+=("$1")
    shift
  fi
done
pdfunite "${reordered[@]}" final.pdf
rm -f piece-*.pdf
unset reordered

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^{If your document has more than 9999 pages adjust the format specifier e.g. %05d}