Example for [Python 2.7]:

---

from pyPdf import PdfFileWriter, PdfFileReader
import StringIO
from reportlab.pdfgen import canvas
from reportlab.lib.pagesizes import letter

packet = StringIO.StringIO()
can = canvas.Canvas(packet, pagesize=letter)
can.drawString(10, 100, "Hello world")
can.save()

#move to the beginning of the StringIO buffer
packet.seek(0)

# create a new PDF with Reportlab
new_pdf = PdfFileReader(packet)
# read your existing PDF
existing_pdf = PdfFileReader(file("original.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("destination.pdf", "wb")
output.write(outputStream)
outputStream.close()

Example for Python 3.x:

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

packet = io.BytesIO()
can = canvas.Canvas(packet, pagesize=letter)
can.drawString(10, 100, "Hello world")
can.save()

#move to the beginning of the StringIO buffer
packet.seek(0)

# create a new PDF with Reportlab
new_pdf = PdfFileReader(packet)
# read your existing PDF
existing_pdf = PdfFileReader(open("original.pdf", "rb"))
output = PdfFileWriter()
# add the "watermark" (which is the new pdf) on the existing page
page = existing_pdf.pages[0]
page.merge_page(new_pdf.pages[0])
output.add_page(page)
# finally, write "output" to a real file
output_stream = open("destination.pdf", "wb")
output.write(output_stream)
output_stream.close()

pdfrw will let you read in pages from an existing PDF and draw them to a reportlab canvas (similar to drawing an image). There are examples for this in the pdfrw examples/rl1 subdirectory on github. Disclaimer: I am the pdfrw author.

cpdf will do the job from the command-line. It isn't python, though (afaik):

cpdf -add-text "Line of text" input.pdf -o output .pdf

Leveraging David Dehghan's answer above, the following works in Python 2.7.13:

from PyPDF2 import PdfFileWriter, PdfFileReader, PdfFileMerger

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(290, 720, "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("original.pdf")
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 = open("destination.pdf", "wb")
output.write(outputStream)
outputStream.close()

2024 update: PyPDF is active again, replacing the need for PyPDF2. Text annotations can be made with the PyPDF package as demonstrated with the FreeText annotation here (code chunk from link quoted below). This can do the trick for simple cases where you need to add text.

### From pypdf docs:
### https://pypdf.readthedocs.io/en/stable/user/adding-pdf-annotations.html#free-text

from pypdf import PdfReader, PdfWriter
from pypdf.annotations import FreeText

# Fill the writer with the pages you want
pdf_path = os.path.join(RESOURCE_ROOT, "crazyones.pdf")
reader = PdfReader(pdf_path)
page = reader.pages[0]
writer = PdfWriter()
writer.add_page(page)

# Create the annotation and add it
annotation = FreeText(
    text="Hello World\nThis is the second line!",
    rect=(50, 550, 200, 650),
    font="Arial",
    bold=True,
    italic=True,
    font_size="20pt",
    font_color="00ff00",
    border_color="0000ff",
    background_color="cdcdcd",
)
writer.add_annotation(page_number=0, annotation=annotation)

# Write the annotated file to disk
with open("annotated-pdf.pdf", "wb") as fp:
    writer.write(fp)

The PyPDF2 as of date of writing has depreciated the PdfFileReader, PdfFileWriter and few other methods and changed it to different names and methods and has also changed methods like getPage() directly to attribute of PdfReader.

Here is a very Simple Class to add text to existing pdf file: (Use is demonstrated at end)

from PyPDF2 import PdfWriter, PdfReader, Transformation
import io
from reportlab.pdfgen.canvas import Canvas

class GenerateFromTemplate:
    def __init__(self,template):
        self.template_pdf = PdfReader(open(template, "rb"))
        self.template_page= self.template_pdf.pages[0]

        self.packet = io.BytesIO()
        self.c = Canvas(self.packet,pagesize=(self.template_page.mediabox.width,self.template_page.mediabox.height))

    
    def addText(self,text,point):
        self.c.drawString(point[0],point[1],text)

    def merge(self):
        self.c.save()
        self.packet.seek(0)
        result_pdf = PdfReader(self.packet)
        result = result_pdf.pages[0]

        self.output = PdfWriter()

        op = Transformation().rotate(0).translate(tx=0, ty=0)
        result.add_transformation(op)
        self.template_page.merge_page(result)
        self.output.add_page(self.template_page)
    
    def generate(self,dest):
        outputStream = open(dest,"wb")
        self.output.write(outputStream)
        outputStream.close()

"""
Use as:
gen = GenerateFromTemplate("template.pdf")
gen.addText("Hello!",(100,200))
gen.addText("PDF!",(100,300))
gen.merge()
gen.generate("Output.pdf")
"""

Hope this helps.

Don't use mergePage, It may not work for some pdfs You should use mergeRotatedTranslatedPage

from PyPDF2 import PdfFileWriter, PdfFileReader
import io
from reportlab.pdfgen.canvas import Canvas

page_to_merge = 0 #Refers to the First page of PDF 
xcoor = 250 #To be changed according to your pdf
ycoor = 650 #To be changed according to your pdf

input_pdf = PdfFileReader(open("Source.pdf", "rb"))
page_count = input_pdf.getNumPages()
inputpdf_page_to_be_merged = input_pdf.getPage(page_to_merge)

packet = io.BytesIO()
c = Canvas(packet,pagesize=(inputpdf_page_to_be_merged.mediaBox.getWidth(),inputpdf_page_to_be_merged.mediaBox.getHeight()))
c.drawString(xcoor,ycoor,"Hello World")
c.save()
packet.seek(0)

overlay_pdf = PdfFileReader(packet)
overlay = overlay_pdf.getPage(0)

output = PdfFileWriter()

for PAGE in range(page_count):
    if PAGE == page_to_merge:
        inputpdf_page_to_be_merged.mergeRotatedTranslatedPage(overlay, 
                inputpdf_page_to_be_merged.get('/Rotate') or 0, 
                overlay.mediaBox.getWidth()/2, overlay.mediaBox.getWidth()/2)
        output.addPage(inputpdf_page_to_be_merged)
    
    else:
        Page_in_pdf = input_pdf.getPage(PAGE)
        output.addPage(Page_in_pdf)

outputStream = open("destination.pdf", "wb")
output.write(outputStream)
outputStream.close()

The combination of pypdf and fpdf can add text as an overlay to an existing PDF.

Ensure both packages are installed: pip install pypdf fpdf2. Tested with fpdf2==2.8.1 and pypdf==5.1.0.

Then adjust the below to your needs:

import io, os

from fpdf import FPDF
from pypdf import PdfReader, PdfWriter

RESOURCE_ROOT = os.path.abspath('c:\\temp\\')
input_path = os.path.join(RESOURCE_ROOT, "inputfile.pdf")
output_path = os.path.join(RESOURCE_ROOT, "outputfile.pdf")

def new_content():
    pdf = FPDF()
    pdf.add_page()
    pdf.set_font("Helvetica", "I", 30)  # Helvetica font, italic, size 30pt
    pdf.text(40, 100, "This is some overlay text")  # X and Y coordinates, text to add
    return io.BytesIO(pdf.output())


reader = PdfReader(input_path)
page_overlay = PdfReader(new_content()).pages[0]
reader.pages[0].merge_page(page2=page_overlay)  # Overlay on page #0 (first page)

writer = PdfWriter()
writer.append_pages_from_reader(reader)
writer.write(output_path)