Hi Ny Emotion,

If you are looking to concatenate / merge two (or more) PDF files, you can simply check the link below (I tested the solution and it works perfectly ; both with Pypdf and PyPDF2) :

https://stackoverflow.com/questions/3444645/merge-pdf-files

If you are looking to concatenate two (or  more) PDF Structure (https://en.wikipedia.org/wiki/PDF , see chapter "Technical Overview"), it's a little bit more tricky but I manage to do that a few days ago.

Below is the code snippet I created to do so (I hope this will be useful to you) :

# Useful librairies
import StringIO
import cStringIO
from openerp.report.pyPdf import PdfFileWriter, PdfFileReader

# "results" is a list containing the PDF structures to merge
output = PdfFileWriter()
for result in results:
    reader = PdfFileReader(cStringIO.StringIO(result))
    for page in range(reader.getNumPages()):
        output.addPage(reader.getPage(page))
s = cStringIO.StringIO()
output.write(s)
return s.getvalue()