On the heels of PDF Generation with Pisa in Django I’ve had the need to pull from a library of pdfs and concatenate based on user selection. At first there seemed to be no good solutions but research and a recommendation by James Tauber (thanks James) turned up pdftk (the pdf toolkit).
The pdf toolkit has some nice features beyond concatenation to inlcude:

• Merge PDF Documents
• Split PDF Pages into a New Document
• Rotate PDF Pages or Documents
• Decrypt Input as Necessary (Password Required)
• Encrypt Output as Desired
• Fill PDF Forms with FDF Data or XFDF Data and/or Flatten Forms
• Apply a Background Watermark or a Foreground Stamp
• Report on PDF Metrics such as Metadata, Bookmarks, and Page Labels
• Update PDF Metadata
• Attach Files to PDF Pages or the PDF Document
• Unpack PDF Attachments
• Burst a PDF Document into Single Pages
• Uncompress and Re-Compress Page Streams
• Repair Corrupted PDF (Where Possible)

I won’t go into all these options. You can see the command reference on the pdftk site.

To do the concatenation in your Django app you can something like this:

import os

def makepdf(request, some_id_todo_something_with):
    os.system("pdftk filename1.pdf filename2.pdf cat output newname.pdf")
    return HttpResponseRedirect(reverse('send_somewhere'))

I’m simply serving them up as static files when they are done.