Add a logo or image watermark to a PDF
An image watermark stamps a picture — usually a company logo, a "DRAFT" graphic, or a signature seal — onto the pages of a PDF so that every copy visibly carries it. This page stamps an uploaded PNG or JPG onto your PDF entirely in your browser: pick a position (corners, center) or tile it across the page, set the stamp width as a percentage of the page width, choose an opacity between 0.05 and 1, and limit it to a page range. Your files never leave your device.
How it works
The tool opens your PDF with pdf-lib and embeds the watermark image exactly once as an image XObject — a self-contained resource object inside the PDF that pages can reference by name. A PNG is stored losslessly (its alpha channel is preserved as a soft mask, which is what keeps transparent logo backgrounds transparent), while a JPG is copied through as a DCT-encoded stream without re-compression, so the stamp loses no quality on the way in.
Each page in your selected range then gets drawing operators appended to its content stream. The stamp width is computed from your percentage setting against that page's own width — 25% on an A4 page is about 149 pt, on a US Letter page about 153 pt — and the height follows from the image's aspect ratio, so the logo is never distorted and mixed page sizes each get a proportionally correct stamp. Opacity is applied through an extended graphics state (ExtGState) with a constant fill alpha between 0.05 and 1; at 0.15, underlying text stays fully readable through the stamp.
In tiled mode the same XObject is drawn repeatedly on a grid with even gaps until the page is covered, which is the classic "confidential wallpaper" effect — and because every tile is a reference to the one embedded image, a page of forty tiles costs no more file size than a single stamp. In positioned mode the image is placed at the corner or center you chose, inset by a small margin from the page edges.
Because the operators are written into the page content itself rather than added as annotations, the watermark prints with the document, survives flattening, and cannot be toggled off in a viewer. Pages outside your range are left completely untouched.
Worked example: branding a 24-page proposal
A consultant wants her 24-page, 1.8 MB proposal PDF branded with the firm's logo, a 900×300 px transparent PNG weighing 64 KB. She sets the position to bottom-right, the width to 18% of the page, and the opacity to 0.9, and limits the range to pages 2-24 so the designed cover page stays clean. On each A4 page the logo renders about 107 pt (38 mm) wide and 36 pt tall, tucked into the footer margin.
The output is 1.87 MB: one 64 KB image XObject plus 23 short content-stream additions. For an internal review copy of the same document, she runs the tool again on the original with a gray "DRAFT" PNG, tiled mode, width 30%, and opacity 0.12 — every page gets a repeating pale stamp that photocopies visibly but leaves 11 pt body text perfectly legible.
Frequently asked questions
Should I use a PNG or a JPG for the watermark?
Use a PNG if your logo needs a transparent background, because JPEG has no alpha channel and would stamp a solid rectangle. A JPG is fine for photographic stamps that fill their whole frame.
Can readers remove the watermark from the output PDF?
Not with a normal viewer — the image is drawn into each page's content stream rather than added as an annotation, so there is no watermark object to click and delete. A determined user with PDF editing software could still reconstruct the page, so treat it as a deterrent, not security.
What opacity should a background logo watermark use?
For a stamp behind or over body text, 0.1 to 0.2 keeps the page readable while remaining clearly visible on screen and in print. Use higher values, around 0.8 to 1, only for corner logos placed in the page margin.
Can I watermark only some pages of the PDF?
Yes — enter a page range such as 2-10 or a list like 1,3,5-12 and only those pages are stamped. Pages outside the range pass through byte-for-byte untouched.
Does adding an image watermark make the PDF much bigger?
The image is embedded once as a shared XObject and referenced from every stamped page, so the file grows by roughly the image's own size regardless of page count. A 60 KB logo adds about 60 KB whether you stamp 5 pages or 500.
Is my document or logo uploaded to a server?
No — both the PDF and the watermark image are read and combined by JavaScript in your browser, and the stamped file is assembled on your device. Nothing is transmitted anywhere.