How to output an info page to pdf

Here's a variant of un2up, which uses Python with the pyPdf library. Note that you need at least version 1.13 (prior versions did not support scaling). Untested.

#!/usr/bin/env python
import copy, math, sys
from pyPdf import PdfFileWriter, PdfFileReader
input = PdfFileReader(sys.stdin)
output = PdfFileWriter()
for p in [input.getPage(i) for i in range(0,input.getNumPages())]:
    p.scaleBy(math.sqrt(2))
    q = copy.copy(p)
    (w, h) = p.mediaBox.upperRight
    p.mediaBox.upperRight = (w/2, h)
    q.mediaBox.upperLeft = (w/2, h)
    output.addPage(p)
    output.addPage(q)
output.write(sys.stdout)

The PyPdf library makes this sort of things easy if you're willing to write a bit of Python. Save the code below in a script called pdf-cat-even (or whatever you like), make it executable (chmod +x pdf-cat-even), and run it as a filter (./pdf-cat-even a.pdf b.pdf >concatenated.pdf). You need pyPdf ≥1.13 for the addBlankPage method.

#!/usr/bin/env python
import copy, sys
from pyPdf import PdfFileWriter, PdfFileReader
output = PdfFileWriter()
output_page_number = 0
alignment = 2           # to align on even pages
for filename in sys.argv[1:]:
    # This code is executed for every file in turn
    input = PdfFileReader(open(filename))
    for p in [input.getPage(i) for i in range(0,input.getNumPages())]:
        # This code is executed for every input page in turn
        output.addPage(p)
        output_page_number += 1
    while output_page_number % alignment != 0:
        output.addBlankPage()
        output_page_number += 1
output.write(sys.stdout)