Watermarks are drawn onto the page as content. That makes them permanent in your copy and removable by anyone with an editor.
Use them to communicate status: draft, confidential, sample, the recipient's name. Do not use them as a security measure, because they are not one.
The classic diagonal DRAFT across a page does a real job. It stops someone circulating a version that was not final, and it makes the document's status obvious at a glance rather than buried in a covering email nobody reads.
A watermark is ordinary page content
Text or an image drawn onto each page, usually at reduced opacity and often rotated. Technically it is no different from any other graphic on the page, which explains both its strengths and its limits.
Because it is page content, it prints, it survives being sent anywhere, and it appears in every reader without depending on annotation support. Because it is page content, it can be removed by anything that can edit page content.
Drawn onto the page like any other text or graphic.
No dependency on the reader supporting a separate layer.
Removable by anyone with an editor.
Keeping the document readable
The usual mistake is a watermark so assertive that the document underneath becomes hard to read, and the usual cause is opacity.
A watermark exists to be noticed rather than to dominate. For a DRAFT or CONFIDENTIAL stamp drawn on top of the page, about 25 percent opacity reads as a mark and leaves the text readable, and that is where the Watermark PDF tool starts its opacity slider. A logo placed behind the content can go fainter, at 10 to 15 percent. At full opacity the mark hides whatever it crosses, which makes it a redaction with a friendly name.
- Opacity25 percent on top, 10 to 15 behind
- Rotation45 degrees diagonal
- ColourGrey rather than black
- PositionCentred on the page
- SizeLarge, spanning the page
- LayerBehind the text where possible
What watermarks are good for
The sensible uses are all about communication rather than control.
Marking status
DRAFT, FOR REVIEW, SUPERSEDED, NOT FOR CIRCULATION. The most valuable use, because it stops honest mistakes: someone finding an old file and acting on it as if it were current.
Marking ownership
A company name or logo on a document that will circulate. It survives forwarding and screenshotting, so the origin stays attached to the content even when the covering message is long gone.
Marking the recipient
Stamping each copy with the name of the person it went to. It does not prevent sharing, and it makes a leaked copy traceable, which changes behaviour more effectively than any restriction.
Image watermarks
A logo works the same way as text and has one additional requirement: transparency. An image with a white background paints a white rectangle across the page, obscuring the content behind it.
Use PNG, which supports transparency properly, and confirm that the background is transparent rather than white. Keep the image large enough to be legible at the opacity you are using, since a small logo at low opacity all but disappears.
Removing a watermark
Worth understanding both because you may need to and because it clarifies what watermarking is worth.
If the watermark is a discrete object on the page, an editor can usually select and delete it. If it has been merged into the page content, removal means editing page content, which is more work and entirely possible. When the goal is that nobody can read something, redacting the sensitive information removes it from the file, which a watermark never does.
Watermark a PDF in your browser
Text or a logo, at any angle, colour and opacity. The file never leaves your device.
Common questions
Using them for what they are good at.
01Does a watermark protect my document?
No. It is ordinary page content and anyone with an editor can remove it. Watermarks communicate status to people reading in good faith, which is useful, but they are not a security measure.
02What opacity should I use?
About 25 percent for a stamp drawn on top, and 10 to 15 percent for a logo placed behind the content. Those figures assume the default grey. A black or strongly coloured mark reads heavier at the same setting, so take it down a step or two if you change the colour.
03Should the watermark go behind or in front of the text?
Behind, for a text document, because it keeps the text crisp. The exception is a scanned page: the whole page is one opaque picture, so a mark placed behind it is hidden completely. For scans, use On top, which is PDFslime's default.
04Can I watermark only some pages?
Yes. PDFslime's Watermark PDF takes a page range such as 2- or 1, 3, 5-9, and stamps only those pages. That lets you mark an appendix as confidential while leaving the cover clean.
05My logo has a white box around it. Why?
The image has an opaque background. Use a PNG with a transparent background rather than a JPEG, which cannot store transparency at all and always fills its background with a solid colour.
06Can watermarks be removed?
Yes, by anyone with a PDF editor. If the point is that some content must not be seen at all, a watermark is the wrong tool: redaction deletes the text and images underneath, while a mark only sits on top of them. Keep an unmarked original either way.