Add Watermark to PDF

Vector stamp vs image composite, and why it matters

When the watermark text is plain ASCII — Latin letters, digits and punctuation — the engine draws it as true vector text objects with Helvetica Bold. The stamp sits on top of every page, rotated to your angle, tiled across the sheet, and respects your opacity. Because it is real vector text, the rest of the document remains exactly as it was: you can still select paragraphs, copy clauses, and search the file. The output file grows by almost nothing.

When the text contains any non-ASCII character — for example Chinese, Cyrillic or accented words that the embedded Helvetica does not cover — the tool switches strategy. Each page is rendered by pdf.js onto a canvas, the watermark is painted over it, and the canvas becomes the new page image. This guarantees the stamp looks correct in any script, but the trade-off is shown to you up front: after the download, every page is a picture and text selection is gone. That is why the inline note appears as soon as a non-ASCII character is typed — use Latin labels like CONFIDENTIAL if searchability matters.

• Angle: 30 degrees is the readable default; 45 looks more formal, flatter angles (10–15) feel subtle, steeper ones (60–90) are aggressive.
• Opacity: 0.25–0.35 is the professional band — visible enough to deter copying, light enough that text underneath stays legible.
• Color: gray is neutral, red reads as "confidential", blue suits corporate review copies, green is common for "sample" or "draft".
• Size: 30–45 pt on A4 keeps the tiled pattern dense without overpowering the layout.

Where watermarking fits in a document workflow

Watermarking is almost always the final step on a master you will circulate, not a step on a file you will keep editing. The safe sequence is: merge all the sections, split out the pages that should not carry the stamp, add page numbers, then watermark, then compress only if you need to email the result. Because the watermark is burned in, always watermark the copy you share — keep the clean master in a folder of its own.

For public-facing versions you may want the opposite protection: a clean PDF with no stamp at all, and smaller. If the source started as photos or scans, the PDF compressor shrinks it; if you need to quote a page outside the document, the PDF to image converter renders the watermarked page to PNG — the stamp travels with the image, which is usually exactly what you want for a preview.

Choosing the right stamp for your audience

The label and the style say who the document is for. DRAFT in light gray at 20 percent opacity signals "work in progress, comments welcome" — it is meant to be polite, not intimidating. CONFIDENTIAL in red at 30 percent says "internal only, do not forward", and the color carries that warning even in a thumbnail preview. SAMPLE in green or gray is the convention on portfolios, pricing sheets and photographer galleries: it discourages direct reuse without looking like an accusation.

Two practical notes. On screen, a watermark at 0.30 reads naturally; when the same file is printed, toner darkens things, so if the PDF will be printed, drop opacity toward 0.20 or the stamp can start to compete with the body text. And remember the watermark is not encryption: it discourages casual copying, it does not stop someone who really wants the text, because a watermarked PDF is still readable underneath. Treat it as labeling, not as DRM — for documents that must not be opened at all, the right tool is password protection in your PDF viewer, not a stamp on the page.

Frequently Asked Questions