Try reportlab.
Also, take a gander at Outputting PDFs.
Edit
I had another thought (but it won't help if you are already have the PDF files, and I like @thraxil's answer better).
Earlier this year I worked on a project where I generated "certificates of completion" for continuing education courses. One of the angles I looked at was trying to generate a PDF directly from an appropriately styled web page (something like a server-side "Print to PDF").
One of the tools I found was wkhtmltopdf. It's a self-contained WebKit browser that will turn a URL into a PDF, and with pretty good results.
The idea is that you use django's template engine to put together a page containing whatever you want (including images), pass it's url to wkhtmltopdf, grab the output and return it to the user.
I liked the approach because it's really simple to implement (just open a pipe), you don't have to worry about keeping the source PDF files accessible to the server, and you can redesign the PDFs by changing the HTML.