add-watermark

Add Diagonal Watermark

Stamp a large rotated text watermark across every page of a PDF.

Inputs

• Input PDF path (required).
• Watermark text (required) — DRAFT, PRIVATE, CONFIDENTIAL, or arbitrary string.
• Color (optional) — default light red (RGB 0.85, 0.2, 0.2 at 25% opacity). Accept named colors or hex.
• Opacity (optional) — default 0.20 (20%).
• Angle (optional) — default 45°.
• Font (optional) — default Helvetica Bold.
• Size (optional) — default auto-fit (≈ page diagonal length / text length × 0.6).
• Output path (optional) — defaults to <input>-<watermark-slug>.pdf.

Tooling

Generate a single-page overlay PDF the same size as the input, then stamp it onto every page using qpdf --overlay.

Overlay generation (Python + ReportLab)

from reportlab.pdfgen import canvas
from reportlab.lib.colors import Color

c = canvas.Canvas("overlay.pdf", pagesize=(width_pt, height_pt))
c.saveState()
c.translate(width_pt/2, height_pt/2)
c.rotate(45)
c.setFillColor(Color(0.85, 0.2, 0.2, alpha=0.20))
c.setFont("Helvetica-Bold", font_size)
c.drawCentredString(0, 0, "DRAFT")
c.restoreState()
c.save()

Stamp onto input

qpdf input.pdf --overlay overlay.pdf -- output.pdf

Workflow

1. Probe page size with pdfinfo.
2. Compute font size to fit (text width ≈ 60% of page diagonal).
3. Generate the overlay.
4. Verification — render page 1 of the output to PNG and inspect:

   gs -sDEVICE=png16m -r100 -o /tmp/wm-check.png -dFirstPage=1 -dLastPage=1 output.pdf
   

   Confirm watermark is visible, centered, at the right angle, body text still readable.
5. Stamp onto all pages.
6. Report and offer to show the user the preview PNG.

Output

Watermarked PDF.

Caveats

• Some viewers (older Acrobat) struggle with PDF transparency; if printing is critical and the printer is old, consider flatten-transparency after watermarking.