How to Add a Watermark to a PDF (Opacity from 15% to 75% Compared, Without Hiding the Text)

I added Confidential and Copy watermarks at different opacities and measured the contrast of the text underneath. 15–25% worked best, and short text stood out more.

Before sending a quote or an internal report as a PDF, you may want to stamp it with a watermark like "Confidential" or "Copy"—and the first question is what opacity to use. Too dark and the text underneath is hard to read; too light and nobody notices it. With PicLever's PDF watermark, I added watermarks to the same document at 15, 25, 50 and 75% opacity and with different text lengths, then measured how much they hid the body text.

Bottom line: body text stayed easy to read under the watermark at 15–25% opacity. At 25%, the contrast of text under the watermark was a comfortable 12.5:1; at 75% it dropped to 2.5:1, making overlapped words hard to read. Keep the text short. Longer text is scaled down to fit the page width, so an 18-character phrase ended up only about 11mm tall.

How much does each opacity hide the text?

A Confidential watermark over 10pt body text at 15%, 25%, 50% and 75% opacity

Opacity	Contrast of text under the watermark	Result
15%	15.6:1	Subtle; text reads normally
25% (default)	12.5:1	Clearly visible; text still easy to read
50%	6.0:1	Overlapped text looks faded
75%	2.5:1	Overlapped text is hard to read

Black text on white is 21:1, and web accessibility guidelines recommend at least 4.5:1 for body text. Up to 50% stayed above that; 75% fell well short.
Use 15–25% for documents read on screen, and 25% for handouts printed in black and white so the mark doesn't disappear. In grayscale, the default purple, gray and red all came out as a similar light gray at 25%.
If you need a stronger mark, stay at or below 50% and check in the preview that it doesn't cover tables or figures.

Longer text makes a smaller watermark

Watermark size by text length on an A4 page, from a 2-character word to a long phrase

Watermark text	Length	Text height (A4 portrait)
사본 (Copy)	2 characters	about 54mm
대외비 (Confidential)	3 characters	about 39mm
제출용 사본 (Copy for submission)	6 characters	about 24mm
CONFIDENTIAL	12 characters	about 13mm
OO은행 제출용 사본 2026.10	18 characters	about 11mm

With the default settings (horizontal, once in the center, 70% size), the watermark is placed horizontally in the center of the page, and the text size is set automatically so the whole phrase fits within that width.
Long phrases with a recipient and date get small and blend into the body text. Shorter wording—around 6 characters, or one short English word—stands out better.
On a landscape page, the same 대외비 watermark came out larger, about 55mm tall.

Orientation, placement and size options

The Confidential watermark placed horizontally in the center, diagonally at 45° in the center, and diagonally at 45° repeated 3×3

Option	Choices	Result (for 대외비)
Orientation	Horizontal · Diagonal ↗ 45° · Diagonal ↘ 45°	45° places it at an angle in an area about 104×99mm
Placement	Once in the center · Repeat 3×3	Repeat covers the whole page with 9 smaller marks
Size	10–100% (default 70%)	Width about 68mm at 40%, 119mm at 70%, 170mm at 100%

To discourage cropping or screenshotting part of a page, use Repeat 3×3—any piece you cut out still carries the watermark.
Repeated marks overlap more of the text, so 15–25% opacity is recommended.

What changes when you add a watermark

File size barely changes. A 5-page report went from 1,599KB to 1,601KB, and a 9.4MB scanned PDF from 9,660KB to 9,664KB.
Body text stays searchable and copyable. The watermark is laid over the page as an image, so it doesn't affect the text layer, and the watermark text itself isn't searchable.
Enter a Page range like "2-3" to add it to those pages only. Leave it blank for all pages.
A watermark is a visible label, not protection. It can be removed in a PDF editor, so don't rely on it to prevent copying or leaks.

Test setup

Samples: a 5-page text-heavy A4 document (10pt body text), a 5-page report with photos (1.6MB), a 1-page landscape schedule, and a 5-page PDF simulating a 300 dpi scan (9.4MB)
Added watermarks with PicLever PDF Watermark at 15, 25, 50 and 75% opacity, in 3 colors (default, gray, red) and with 5 different phrases · Tested 2026-10-01; orientation, placement and size options checked 2026-10-08
Text contrast was calculated from the brightness of body text and its surrounding background where the watermark overlaps, rendered in grayscale at 200 dpi.

Adding a watermark to a PDF with PicLever

In PDF Watermark, click Choose PDF and add your file (up to 50MB and 200 pages).
Pick a Color. The default color prints as light gray in black and white.
Type short Watermark text such as "CONFIDENTIAL" or "COPY". Both English and Korean work.
Choose the Orientation (horizontal or diagonal), Placement (once in the center or repeat 3×3) and Size. The defaults (horizontal, center, 70%) are a good start.
Leave Opacity at 25% (the default), and enter a Page range if you only want it on some pages.
Click Create result, check in the PDF preview that the text isn't covered, then save the result.

FAQ

Q. Can I add the watermark diagonally?
Yes. Choose Diagonal ↗ 45° or ↘ 45° under Orientation. To cover the whole page with several marks, set Placement to Repeat 3×3.

Q. What happens if I add a watermark to a PDF that already has one?
The new watermark is layered on top of the old one. To change the wording, start again from the original PDF without a watermark.

Q. How do I add the same watermark to several PDFs?
Combine them first with Merge PDF, add the watermark once, and split them again with Split PDF if needed.