[Python] Downloading ReportLab PDF with Flask - Python

I have a function which generates a PDF and I have a Flask website. I would like to combine the two so that when you visit the site, a PDF is generated and downloaded. I am working on combining various bits of code that I don't fully understand. The PDF is generated and downloaded but fails to ever load when I try to open it. What am I missing?

import cStringIO
from reportlab.lib.enums import TA_JUSTIFY
from reportlab.lib.pagesizes import letter
from reportlab.platypus import SimpleDocTemplate, Paragraph
from reportlab.lib.styles import getSampleStyleSheet, ParagraphStyle
from flask import make_response, Flask
from reportlab.pdfgen import canvas

app = Flask(__name__)
@app.route('/')
def pdf():

output = cStringIO.StringIO()
doc = SimpleDocTemplate("test.pdf",pagesize=letter)
Story=[]

styles=getSampleStyleSheet()
styles.add(ParagraphStyle(name='Justify', alignment=TA_JUSTIFY))
ptext = '<font size=12>test</font>'

Story.append(Paragraph(ptext, styles["Justify"]))

doc.build(Story)
pdf_out = output.getvalue()
output.close()

response = make_response(pdf_out)
response.headers['Content-Disposition'] = "attachment; filename='test.pdf"
response.mimetype = 'application/pdf'
return response
app.run()

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