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CALCPARK

PDF Watermark

Stamp a text or image watermark on every page of a PDF, or on selected pages. Set opacity, angle, size, colour and position with a live preview – all in your browser.

Files stay on your device
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How to use it

  1. Drop the PDF onto the box. The first page appears in the preview as soon as it has been rendered.
  2. Choose “Text” and type the words (e.g. CONFIDENTIAL, DRAFT, a name or a date), or “Image or logo” and drop a PNG with transparency.
  3. Adjust size (or logo width), colour, opacity, rotation and position while watching the preview. Limit the pages if only some should carry the mark.
  4. Press “Add watermark” and download. The original file is not modified.

Example

“DRAFT” in Helvetica Bold at 96 pt, 30% grey, rotated 45° and centred on an A4 page: the text is about 325 pt wide and 69 pt high (cap height 0.718 × 96), so its rotated bounding box is 279 × 279 pt and its centre sits at (297.6, 420.9) pt – the middle of the page. A 400 × 100 px logo at 35% width on the same page is drawn 208 × 52 pt in the bottom-right corner, 24 pt from the edges.

How the watermark is placed

pdf-lib opens the file and, for each selected page, appends a drawText or drawImageoperation with the chosen opacity (an ExtGState with constant alpha) and rotation. pdf-lib rotates around the object’s own origin (bottom-left corner), so the tool first computes the bounding box of the rotated watermark, anchors that box at the chosen position with a 24 pt margin, and converts the box centre back into an origin:

origin = centre + R(θ) · (−w/2, −h/2)

For text, w is the string width from the font’s metrics and h the cap height (0.718 × size for Helvetica, 0.676 for Times, 0.562 for Courier). For images w is the chosen share of the page width and h follows from the aspect ratio. Pages with a /Rotate entry are handled in their displayed orientation, so a landscape scan gets the same diagonal as an upright page. The preview draws the same geometry on a canvas over a pdf.js rendering of page 1.

Limits

  • Text uses the PDF standard fonts (Helvetica, Times, Courier in bold), which cover Latin-1 plus common typographic symbols – no Cyrillic, Greek or CJK text.
  • The watermark is drawn above existing content; it does not go behind it and it is not a separate layer, so a viewer cannot switch it off.
  • It is a visual mark, not a security feature: anyone with an editor can remove it.
  • Encrypted PDFs are refused; unlock them first.

Sources: pdf-lib – drawText / drawImage options, PDF 32000-1:2008 – 8.4.5 Graphics state parameter dictionaries (CA/ca), Adobe Font Metrics for the standard 14 fonts.

Questions

Can the watermark be removed later?
Yes, by anyone with a PDF editor, because it is ordinary page content. Use it to label documents, not to protect them.
Why does the preview look slightly different from the result?
The preview is drawn with your system's sans-serif, serif or monospace font, while the PDF embeds the standard Helvetica, Times or Courier metrics. Position, size, angle and opacity are computed identically.
How do I put a logo in the corner of every page?
Choose "Image or logo", drop a PNG, set the width to 15–25%, rotation 0 and position "Bottom right" (or any corner). Keep the opacity at 100% if the logo should not look faded.
Can I watermark only some pages?
Yes – type a selection in "Pages", for example 2- to skip the cover or 1, 5, 9 for specific pages.