UPDATE for 2021:

Since this answer is over half a decade old, some new solutions have become available. These days, I tend to use WeasyPrint, which has the additional benefit of being BSD licensed instead of LGPL. It is a tad slower, however.

https://weasyprint.org/

ORIGINAL ANSWER:

I'd recommend using wkhtmltopdf.

The short answer? On Ubuntu, install a binary:

apt-get install wkhtmltopdf

On CentOS / RedHat:

yum install wkhtmltox-0.12.2.1_linux-centos6-amd64.rpm

Then pip install a Python package:

pip install pdfkit

Then the code:

import pdfkit
 
input_filename = 'README.html'
output_filename = 'README.pdf'
 
with open(input_filename, 'r') as f:
    html_text = f.read()
 
pdfkit.from_string(html_text, output_filename)

For the long answer and details, I put together a blog post:

https://www.pyphilly.org/generating-pdf-markdown-or-html/

That should take care of the PDF creation; you'll have to decide how you want to handle the download. Good luck!

Try:

# code...
page = renderPDF.drawToFile(drawing, 'example.pdf', 'lineplot with dates')
response = HttpResponse(page, mimetype='application/pdf')

I'm using reportlab.pdfgen.canvas and it works great.
Keep in mind that it's client's browser (and/or OS) job to open your file in browser after recognizing it as a PDF.

EDIT:
That's how I present invoices on some Web2Print solution:

from django.http import HttpResponse
from reportlab.pdfgen import canvas

def invoice_to_response(request, invoice):
    response = HttpResponse(mimetype='application/pdf')
    p = canvas.Canvas(response, pagesize='A4', pageCompression=0)
    # here I draw on 'p' like p.setFillColor(black)
    p.showPage()
    p.save()
    return response

Using ReportLab

try:
    from cStringIO import StringIO
except ImportError:
    from StringIO import StringIO
from reportlab.pdfgen import canvas
from reportlab.lib.pagesizes import letter, A4
from reportlab.lib.units import inch

def createPDF(request):
 x=100
 y=100
 buffer=StringIO()
 p=canvas.Canvas(buffer,pagesize=letter)
 p.drawString(x,y,"HELLOWORLD")
 p.showPage()
 p.save() 
 pdf=buffer.getvalue()
 buffer.close() 
 return pdf

def someView(request):
 EmailMsg=mail.EmailMessage(YourSubject,YourEmailBodyCopy,'[email protected]',["[email protected]"],headers={'Reply-To':'[email protected]'})
 pdf=createPDF(request)
 EmailMsg.attach('yourChoosenFileName.pdf',pdf,'application/pdf')
 EmailMsg.send()

Works perfectly!!