Add text to existing PDF document in Python

Question

I'm trying to convert a pdf to the same size as my pdf which is an A4 page.

convert my_pdf.pdf -density 300x300 -page A4 my_png.png

The resulting png file, however, is 595px × 842px which should be the resolution at 72 dpi. I was thinking of using PIL to write some text on some of the pdf fields and convert it back to PDF. But currently the image is coming out wrong.

Edit: I was approaching the problem from the wrong angle. The correct approach didn't include imagemagick at all.

Answer 1

After searching around some I finally found the solution: It turns out that this was the correct approach after all. Yet, i feel that it wasn't verbose enough. It appears that the poster probably took it from here (same variable names etc).

The idea: create new blank PDF with Reportlab which only contains a text string. Then merge/add it as a watermark using pyPdf.

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()

Hope this helps somebody else.

Answer 2

I just tried the solution above, but I had quite some troubles to get it running in Python3. So, I would like to share my modifications. The adapted code looks as follows:

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

packet = io.BytesIO()

# 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(open("mypdf.pdf", "rb"))
output = PdfFileWriter()
# add the "watermark" (which is the new pdf) on the existing page
page = existing_pdf.getPage(0)
page2 = new_pdf.getPage(0)
page.mergePage(page2)
output.addPage(page)
# finally, write "output" to a real file
outputStream = open("newpdf.pdf", "wb")
output.write(outputStream)
outputStream.close()

Now the page.mergePage throws an error. Turns out to be a porting error in pypdf2. Please refer to this question for the solution: Porting to Python3: PyPDF2 mergePage() gives TypeError