How it works
We embed your image with pdf-lib's embedPng/embedJpg and stamp it onto every page using page.drawImage with your chosen opacity.
FAQ
Should I use PNG or JPG? PNG if your logo has transparency (most logos do). JPG only if your watermark is a photograph or you do not need a transparent background.
How does opacity interact with PNG transparency? They multiply. A logo with 100% opaque pixels at 40% opacity in this tool comes out at 40%. A PNG that already has 50% opacity at 50% opacity here comes out at 25%.
Why is the watermark sized in points and not pixels? PDFs use points (1/72 inch) as their fundamental unit. 150 points is roughly 2 inches wide — large enough to be visible, small enough to fit comfortably in a corner.