Edit an existing PDF using Python

editExistingPdf.py

from pyPdf import PdfFileWriter, PdfFileReader
import StringIO
from reportlab.pdfgen import canvas
from reportlab.lib.pagesizes import letter
packet = StringIO.StringIO()
# create a new PDF with Reportlab
can = canvas.Canvas(packet, pagesize=letter)
can.drawString(100,100, "Hello world")
can.save()

#move to the beginning of the StringIO buffer
packet.seek(0)
new_pdf = PdfFileReader(packet)
# read your existing PDF
existing_pdf = PdfFileReader(file("mypdf.pdf", "rb"))
output = PdfFileWriter()
# add the "watermark" (which is the new pdf) on the existing page
page = existing_pdf.getPage(0)
page.mergePage(new_pdf.getPage(0))
output.addPage(page)
# finally, write "output" to a real file
outputStream = file("/home/joe/newpdf.pdf", "wb")
output.write(outputStream)
outputStream.close()

A new version:

from PyPDF2 import PdfFileWriter, PdfFileReader
from io import BytesIO
from reportlab.pdfgen import canvas
from reportlab.lib.pagesizes import A4

buffer = BytesIO()

# create a new PDF with Reportlab
p = canvas.Canvas(buffer, pagesize=A4)
p.drawString(100, 100, "holaaaaaaaaaaaa")
p.showPage()
p.save()

#move to the beginning of the StringIO buffer
buffer.seek(0)
newPdf = PdfFileReader(buffer)

#######DEBUG NEW PDF created#############
pdf1 = buffer.getvalue()
open('pdf1.pdf', 'wb').write(pdf1)
#########################################
# read your existing PDF
existingPdf = PdfFileReader(open('plantilla.pdf', 'rb'))
output = PdfFileWriter()
# add the "watermark" (which is the new pdf) on the existing page
page = existingPdf.getPage(0)
page.mergePage(newPdf.getPage(0))
output.addPage(page)
# finally, write "output" to a real file
outputStream = open('output.pdf', 'wb')
output.write(outputStream)
outputStream.close()

What if I want to add watermark or some barcode to specify page in pdf, and save it as a one pdf (with e.g. barcode on second page?). How to multiplay pages here?
Thanks

what if I want to replace some text in the pdf?

This is not possible with PDFs. See this answer from Stackoverflow.