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What you will build
The workflow uses two focused libraries:
- ReportLab creates a PDF page containing your text. Its canvas uses Cartesian coordinates with the origin at the lower-left corner.
- pypdf reads the input PDF, merges the watermark page, and writes the result.
The watermark is normally generated once and reused for every page. The output is a new PDF; the source file is not modified in place.
Install the dependencies
Use a virtual environment when possible, then install both packages:
python -m pip install reportlab pypdfFor production applications, pin and verify the versions installed in your deployment because PDF behavior can change between library releases.
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Complete example: watermark every page
This script places a translucent “CONFIDENTIAL” label near the lower-left corner and puts it behind existing page content.
from io import BytesIO
from reportlab.lib.pagesizes import letter
from reportlab.pdfgen import canvas
from pypdf import PdfReader, PdfWriter
INPUT = "input.pdf"
OUTPUT = "output-watermarked.pdf"
# Build a one-page PDF containing the watermark text.
watermark_bytes = BytesIO()
wm = canvas.Canvas(watermark_bytes, pagesize=letter)
wm.setFont("Helvetica", 36)
# setFillAlpha is available in ReportLab versions that expose transparency.
# Remove this line if your installed version does not support it.
wm.setFillAlpha(0.25)
wm.drawString(72, 72, "CONFIDENTIAL")
wm.showPage()
wm.save()
watermark_bytes.seek(0)
watermark_page = PdfReader(watermark_bytes).pages[0]
reader = PdfReader(INPUT)
writer = PdfWriter()
for page in reader.pages:
# False places the watermark under the original page content.
page.merge_page(watermark_page, over=False)
writer.add_page(page)
with open(OUTPUT, "wb") as output:
writer.write(output)
print(f"Wrote {OUTPUT}")
Run it with python watermark.py. Open the resulting file in a PDF viewer and check several pages, including pages with images, tables, and rotated content.
Put the text above the PDF content
A background watermark can disappear where the page has an opaque white rectangle. Use a foreground merge when the label must remain visible:
for page in reader.pages:
page.merge_page(watermark_page, over=True)
writer.add_page(page)
In pypdf terminology, a stamp is placed on top, while a watermark is placed in the background. The merge operation is otherwise the same. Omitting over uses the overlay behavior provided by the installed pypdf version, so specify it explicitly when layer order matters.
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Control text, color, opacity, and position
Change the wording and font
ReportLab supplies the text controls. For example:
wm.setFont("Helvetica-Bold", 24)
wm.setFillColorRGB(0.8, 0.1, 0.1)
wm.drawString(72, 100, "DRAFT — DO NOT DISTRIBUTE")
Built-in fonts such as Helvetica are convenient. If you need a specific typeface, register a supported font with ReportLab and confirm that the font is embedded as required by your distribution or archival policy.
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Use a centered or diagonal watermark
A plain merge_page preserves the watermark page’s original geometry. For scaling, translation, or rotation, use merge_transformed_page and a Transformation:
from pypdf import Transformation
transform = (
Transformation()
.rotate(45)
.scale(1.2)
.translate(tx=180, ty=250)
)
for page in reader.pages:
page.merge_transformed_page(
watermark_page,
transform,
over=False,
)
writer.add_page(page)
Adjust the translation and scale for the watermark page size and the target page’s media box. Rotation is in degrees. Render a test page before applying a transform to a large batch because coordinate systems and page boxes affect the visible result.
Handle page rotation correctly
Some PDFs store rotation as page metadata rather than rotating the page’s content stream. If a merge appears sideways or is positioned unexpectedly, transfer the rotation into the content before merging:
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for page in reader.pages:
page.transfer_rotation_to_content()
page.merge_page(watermark_page, over=False)
writer.add_page(page)
Do this only when needed. A document with mixed portrait and landscape pages may require inspecting each page’s rotation and geometry, then creating or transforming a watermark that fits that page.
Support mixed page sizes
The minimal example creates a letter-sized watermark. That is suitable only when the destination pages use compatible dimensions. PDFs often mix letter, A4, legal, portrait, and landscape pages.
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For reliable placement:
- Read each page’s media box or crop box.
- Create a watermark canvas with matching width and height, or use a transformation that scales and translates the reusable watermark.
- Choose coordinates relative to the page dimensions rather than assuming 612 × 792 points.
- Test pages with different boxes and orientations in the same file.
One point equals 1/72 inch in the PDF coordinate system. The default ReportLab origin remains the lower-left corner, so increasing x moves right and increasing y moves up.
Generate a watermark for each page size
When exact geometry matters, create a watermark page inside the loop. This example uses each page’s width and height and places centered text:
from io import BytesIO
from reportlab.pdfgen import canvas
from pypdf import PdfReader, PdfWriter
reader = PdfReader("input.pdf")
writer = PdfWriter()
for page in reader.pages:
width = float(page.mediabox.width)
height = float(page.mediabox.height)
stream = BytesIO()
wm = canvas.Canvas(stream, pagesize=(width, height))
wm.setFont("Helvetica", 30)
wm.setFillAlpha(0.20)
text = "CONFIDENTIAL"
text_width = wm.stringWidth(text, "Helvetica", 30)
wm.drawString((width - text_width) / 2, height / 2, text)
wm.showPage()
wm.save()
stream.seek(0)
watermark_page = PdfReader(stream).pages[0]
page.merge_page(watermark_page, over=False)
writer.add_page(page)
with open("output-watermarked.pdf", "wb") as output:
writer.write(output)
This approach costs more work per page but avoids applying letter-sized coordinates to an A4 or landscape page.
Transparency and rendering caveats
setFillAlpha is optional and version-dependent. Check the ReportLab version installed in your environment before relying on it. If transparency is unavailable, use a lighter fill color or omit the call. PDF viewers can render transparency, fonts, and overlaid graphics differently, so verify the actual output in the viewers and print workflows that matter to you.
Troubleshooting
ModuleNotFoundError
The interpreter running the script does not have the packages. Install them with that interpreter, for example python -m pip install reportlab pypdf, and check that your IDE uses the same virtual environment.
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The watermark is hidden
Opaque page content may cover an underlay. Change over=False to over=True, reduce the watermark’s opacity, or move it into an unused margin.
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The watermark is off the page
ReportLab coordinates start at the lower-left, not the upper-left. Check the page’s width and height and avoid hard-coded letter dimensions for mixed-size documents.
Rotated pages look wrong
Call transfer_rotation_to_content() before merging, then test both portrait and landscape pages. Do not apply it blindly if your input has already-normalized content.
The output is blank or corrupt
Ensure the canvas calls showPage() and save(), rewind every BytesIO object with seek(0), and close the output through writer.write(). Write to a new path while debugging so the original remains available.
Some pages have no visible watermark
Inspect page boxes, clipping, transparency, and layer order. A watermark generated for one page size can fall outside another page’s visible area. Generate per-page watermarks or use a page-specific transformation.
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Performance, reliability, and file handling
The reusable-watermark version keeps the in-memory watermark small and avoids regenerating it for every page. The per-page version is preferable when geometry differs. For large files, process one input at a time, write to a temporary output, and replace the destination only after a successful write. Keep the source PDF until you have validated the output.
Watermarking changes the PDF’s content streams; it does not guarantee that text cannot be removed. If the watermark is a legal or compliance control, combine it with the document-management and access controls required by your policy. Also check whether annotations, forms, encryption, bookmarks, and digital signatures need special handling in your pypdf version before batch processing.
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- Install and pin compatible ReportLab and pypdf versions.
- Create the watermark canvas, call
showPage(),save(), and rewind the stream. - Use
over=Falsefor a background watermark andover=Truefor a foreground stamp. - Use
Transformationfor scale, translation, or rotation. - Normalize problematic page rotation with
transfer_rotation_to_content(). - Match watermark geometry to mixed page sizes.
- Render and inspect representative output pages before replacing the source.
Frequently Asked Questions
Can I watermark only selected pages?
Yes. Iterate with an index and merge the watermark only when that index is in your selected set; add every other page unchanged.
Does this permanently flatten the watermark?
The watermark becomes part of the page content stream, but PDF editing tools may still remove or cover it. It is not a security boundary by itself.
Why use a separate watermark PDF?
ReportLab handles drawing and typography, while pypdf handles PDF reading, page merging, and writing. The separate page also makes the same design reusable across pages.
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