I have an imposed document: there are 4 × n A4 pages on the n sheets. I put them into a roller image scanner and receive one 2 × n paged PDF document (A3).

If, say, n = 3, then I've got the following sequence of A3 pages in my PDF:

• page one: page 12 (on the left) and page 1 of the original document
• page two: p.2 and p.11 of the original document
• page three: p.10 and p.3…
• … and so on until…
• page six: p.6 and p.7 of the original document

Question: how can I reconstruct the original sequence of pages in one PDF file of the A4 format? I.e. I want to do this:

--A3--         --A4--
[12| 1]         [1]
[ 2|11]         [2]
[10| 3]    ⇒    [3]…             … 
[ 6| 7]         [6][7]… [12]

In linux I usually use pdftk or pdftops-like console utilities for this kind of cases, but I cannot figure out how to use them for my current purpose.

After a while I found this thread and tuned the code a bit:

import copy
import sys
import math
import pyPdfdef split_pages(src, dst):src_f = file(src, 'r+b')dst_f = file(dst, 'w+b')input_PDF = pyPdf.PdfFileReader(src_f)num_pages = input_PDF.getNumPages()first_half, second_half = [], []for i in range(num_pages):p = input_PDF.getPage(i)q = copy.copy(p)q.mediaBox = copy.copy(p.mediaBox)x1, x2 = p.mediaBox.lowerLeftx3, x4 = p.mediaBox.upperRightx1, x2 = math.floor(x1), math.floor(x2)x3, x4 = math.floor(x3), math.floor(x4)x5, x6 = math.floor(x3/2), math.floor(x4/2)if x3 > x4:# horizontalp.mediaBox.upperRight = (x5, x4)p.mediaBox.lowerLeft = (x1, x2)q.mediaBox.upperRight = (x3, x4)q.mediaBox.lowerLeft = (x5, x2)else:# verticalp.mediaBox.upperRight = (x3, x4)p.mediaBox.lowerLeft = (x1, x6)q.mediaBox.upperRight = (x3, x6)q.mediaBox.lowerLeft = (x1, x2)if i in range(1,num_pages+1,2):first_half += [p]second_half += [q]else:first_half += [q]second_half += [p]output = pyPdf.PdfFileWriter()for page in first_half + second_half[::-1]:output.addPage(page)output.write(dst_f)src_f.close()dst_f.close()if len(sys.argv) < 3:print("\nusage:\n$ python reverse_impose.py input.pdf output.pdf")sys.exit()input_file = sys.argv[1]
output_file = sys.argv[2]split_pages(input_file,output_file)

See this gist.