Can I put text like “CONFIDENTIAL” on every page?

Yes. Write it in “Watermark text” and leave “Pages to stamp” on “All pages”. It goes in the same spot on every page.

How do I put the watermark in a corner?

Leave “Position” on “Free” and drag it in the preview, or pick a spot in “Quick align”. You can also type numbers in “X” and “Y”.

This tool has no way to remove it. Your original PDF is unchanged, so use the file from before you added the watermark.

Watermark

Stamp text or an image over the pages. Your file never leaves this device.

PDF

How to use Watermark

Stamps text or an image over the pages of a PDF. You can put it on every page or only some, in one spot or tiled across the page.

Drop a PDF onto the screen, or click to choose one.
Under “Content”, set “Watermark type” to “Text” or “Image”. For text, write the “Watermark text”. For an image, choose a PNG or JPG file. It starts on “Text”, with “CONFIDENTIAL”.
Under “Look”, set the “Opacity” and “Rotation” (0°, 45°, 90°). For text you also pick a “Color” (Black, Gray, Red, Blue). It starts on gray, 30% opacity and 45°.
Under “Placement”, set “Position” to “Free” or “Tile”. With “Free”, drag the watermark in the preview and resize it from a corner. “Pages to stamp” is “All pages” or “Range”; for a range, write the pages in “Pages”, like “1-3, 5”.
Click “Add watermark”, then “Save”. The file name is the original name plus “-watermarked”.

Text size runs from 8 pt to 200 pt (it starts at 64 pt). Image size runs from 5% to 100% of the page width (it starts at 40%).
Text with Latin letters only needs no font download. Korean, Japanese and Chinese text downloads a font (2.7–10.6 MB) the first time. Korean mixed with Chinese characters or kana in one text shows as boxes.
The text is real text in the file, so it can be selected and searched.
Until you resize it yourself in “Free”, a watermark that would fill more than 90% of the page is shrunk to fit within that. “Tile” can be cut off at the page edges.
A stamped watermark can be removed again with a PDF editor. It is not a security feature.
Files are processed in your browser and are not uploaded to a server.

Can I put text like “CONFIDENTIAL” on every page?: Yes. Write it in “Watermark text” and leave “Pages to stamp” on “All pages”. It goes in the same spot on every page.
How do I put the watermark in a corner?: Leave “Position” on “Free” and drag it in the preview, or pick a spot in “Quick align”. You can also type numbers in “X” and “Y”.
Can I remove the watermark later?: This tool has no way to remove it. Your original PDF is unchanged, so use the file from before you added the watermark.