Add a Watermark to a PDF
Adding a watermark to a PDF stamps a repeated marking, typically a short diagonal text such as CONFIDENTIAL or DRAFT, onto every page so the document's status is visible on screen and in print. This tool draws the text directly into each page's content stream at an opacity you choose, entirely in your browser, with no upload. Because the stamp becomes part of the page content, it prints with the document and survives every standard viewer.
How it works
Every PDF page is drawn by a content stream: a list of operators that place text, lines, and images in a coordinate space measured in points (72 to the inch, origin at the bottom-left corner). Watermarking appends new operators to that list. For each page, the tool computes the page's dimensions from its media box, then writes a text-showing sequence that positions your watermark string at the page's center and rotates it diagonally — the classic corner-to-corner angle is atan(height/width), about 40 degrees on A4 and Letter portrait pages.
Opacity is the technically interesting part. PDF text has no per-character alpha; translucency comes from an extended graphics state (an ExtGState dictionary) whose CA and ca entries set stroke and fill alpha. The tool registers one such graphics state in the document, then wraps each stamp in q … Q operators (save and restore graphics state) so the transparency applies to the watermark alone and cannot leak into the page's existing content. The watermark text is set in a standard base font (Helvetica), embedded once and shared by every page, which is why the size cost of stamping a 200-page file is barely more than stamping a 2-page one.
Because the operators are appended after the existing content, the watermark renders on top of whatever the page already shows — text, tables, images. That is the behavior you want for a status stamp. It also means the stamp is page content, not an annotation: viewers cannot hide it with an "annotations off" toggle, flattening is unnecessary, and printing always includes it. The trade-off is honesty about security: a determined user with PDF editing software can locate the appended operators and delete them, so a watermark labels a document; it does not protect it. For genuine content protection, redaction or encryption are the right tools.
Worked example: marking a draft contract before review
A 14-page services agreement, msa-v3.pdf at 486 KB, is going to a counterparty for comments and must be unmistakably marked as not final. Stamping the text DRAFT — NOT FOR SIGNATURE at 0.15 opacity, rotated diagonally, takes under a second in the browser. The output file is 489 KB — the 3 KB difference is one Helvetica font resource, one graphics-state dictionary, and fourteen short operator sequences. Zooming to 400% confirms the underlying 10-point body text remains fully readable through the gray stamp, and a test print on a mono laser printer shows the watermark at a legible mid-gray on all 14 pages.
Frequently asked questions
Can a watermark added this way be removed later?
With effort, yes. The watermark becomes a drawing operation in the page's content stream, so PDF editing software that can dissect content streams can strip it out. Treat a text watermark as a clear label, not as a security measure.
Will the watermark appear when the PDF is printed?
Yes. Because the watermark is written into the page content itself rather than added as a removable annotation, every viewer and printer renders it exactly like the rest of the page.
What opacity should I use for a PDF watermark?
Between 0.10 and 0.25 works for most documents: visible at a glance but light enough to leave body text readable. Go darker only for scans or image-heavy pages where a faint stamp would disappear.
Does watermarking change the PDF file size?
Only slightly. The tool adds a font resource plus a few drawing operators per page, typically well under 20 KB in total for the whole document. A 5 MB report stays effectively 5 MB.
Is my document uploaded when I add a watermark here?
No. The PDF is parsed, stamped, and re-saved by JavaScript running in your browser, and the finished file is handed to you as a download from memory. Nothing is sent over the network.
Can I watermark only some pages of the PDF?
This tool stamps every page, which is what CONFIDENTIAL and DRAFT markings almost always require. If you need selective stamping, split the PDF first, watermark one part, and merge the parts back together.