How to watermark a PDF, and what a watermark can and cannot stop

A watermark answers one question, what is this copy, and it answers it on every page, in every reader and on paper. It does not decide who may read or reuse the document. Picking settings that leave the page readable, and being clear about that limit before you send the file, is the whole job.

What a watermark is, in PDF terms

A watermark is text drawn across the content of every page at a chosen angle and opacity. The tool’s watermark function in src/lib/pdf/ops.ts sets it in Helvetica Bold, in a fixed light grey of 60% in each colour channel (0.6 × 255 = 153, which is #999999), positioned from the middle of each page. It is added after the page’s existing content, so it sits on top of what is already there, and the opacity is what lets the words underneath show through.

Watermarking a PDF step by step

1. Open Watermark PDF and choose Add a PDF.
2. Type the mark into Text. It starts as DRAFT. Shorter is better: a long string at an angle collides with the layout on every page at once.
3. Set Size, anywhere from 12 to 140 points. It starts at 60.
4. Set Opacity, from 0.05 to 1 in steps of 0.05. It starts at 0.15.
5. Set Angle, from 0° to 90° in steps of 5°. It starts at 45°, rising from lower left to upper right.
6. Look at the thumbnails. The mark on them is drawn by the browser as an approximation, so make the final judgement on the downloaded file, on the page with the most text.
7. Press Watermark PDF and download the copy, which has -watermarked added to its name.

Empty the text box and the tool refuses with “Type the watermark text first.” rather than handing back a file that looks unchanged.

Choosing opacity, size and angle

Control	Starts at	Range	What to aim for
Opacity	0.15	0.05 to 1, in 0.05 steps	Clearly visible, still readable underneath
Size	60 pt	12 to 140 pt	Readable at a glance, well inside the page edges
Angle	45°	0° to 90°, in 5° steps	The starting diagonal is the usual draft mark
Text	DRAFT	Any text	One word, or a recipient’s name

Opacity is the setting that makes or ruins the page. Around 0.15 is the usual choice. Above about 0.3 the mark starts competing with the text for attention, and below about 0.08 it tends to vanish on paper unless the printer is a good laser. With the slider moving in 0.05 steps, only its lowest position, 0.05, falls under that floor, and the usable band from 0.10 to 0.30 is five positions: 0.10, 0.15, 0.20, 0.25 and 0.30.

Size is not adjusted for you. The text’s width is worked out at the size you choose and nothing shrinks it to fit, so a long name at a large size runs past the edge and is cut off. Lower the size or shorten the text, then check a page.

What it can and cannot stop

	Watermark	Password protection
What it does	Marks status or ownership visually	Restricts opening the document
Survives printing	Yes	Not applicable
Stops reuse	No	Until the password is shared
Removable	By anyone editing the PDF	Only with the password

The mark is part of the page, which is why it shows in every reader and on every print. That is also why anyone comfortable editing a PDF’s internals can find it and delete it. The words underneath remain real text, so they can still be selected, copied and extracted; a watermark changes how a copy looks, not what can be taken from it. Because the mark is itself drawn as text, expect the word to turn up in text copied or extracted from the page as well. Running the result through PDF to text shows exactly what a recipient could lift.

Its real value is social. “DRAFT” stops a draft being mistaken for the final version; a recipient’s name on every page discourages casual forwarding, because the copy says where it came from.

Watermark first, password last

If the PDF you add is password-protected, the tool asks for the password, works on a decrypted copy and returns a watermarked file with no password at all. Every editing tool on the site treats protected input this way. When a document needs both, add the watermark first, then lock the watermarked copy with Protect PDF.

What the tool does not do

It stamps text only, in one colour, on every page. A logo watermark is not offered: an image has to be embedded once and referenced from each page to avoid multiplying the file size, and that is worth doing properly rather than quickly. There is no option to skip pages, so a cover that must stay clean has to be split off with Split PDF first and merged back afterwards.

Other ways to watermark a PDF

Neither macOS Preview nor Windows has a built-in watermark feature that we can point you to, and none of the command-line tools we have tested for these guides adds one. Rather than guess at a route, this guide leaves it there.

Seeing that nothing was sent

The drop zone says the file never leaves the tab, and it is quick to check. Add the PDF, disconnect from the network and press the button: the watermarked copy still downloads. With the Network tab of the developer tools open, you will see the tool’s PDF code arrive when the first file is added, and no request going out with your document in it.

Mark the copy

Watermark PDF is for saying what a copy is. If you need to decide who can open it, that is a password, and how to password-protect a PDF without uploading it covers the cipher, the two kinds of password and the order to do things in.