Where the white box comes from
You sign a sheet of paper and scan it. The scanner records the ink as dark pixels and the paper as white ones, and white pixels are as solid as any other colour. Drop that file into Word and the paper comes with it.
What you want is the ink and nothing else. That means a PNG, where the paper has been set to fully transparent and the ink left alone, with the soft grey edges of the pen strokes kept as partly transparent so they don’t turn into a jagged outline.
Getting a scan worth cutting out
Sign in black or dark blue, on plain unlined paper, and press harder than feels natural. A thin grey signature gives the model very little to hold on to.
A phone photo works as well as a scanner if the light is even. Flat daylight, no flash, and keep your own shadow off the page. If half the sheet is in shade, that shaded half reads as grey rather than white and some of it will survive the cut.
Into Word, Excel or a PDF
Drop the scan above and download the PNG. In Word: Insert → Pictures → This Device, then set Wrap Text to In Front of Text. That last step is the one people miss, and without it the signature can’t sit on top of the line you want it on.
Excel and PowerPoint work the same way. For a PDF, place it as an image stamp in whatever editor you use. The transparency carries through all of them, because it lives in the file.
If the ink comes out looking thin or broken up, open More options and push the Edges grow slider out by a pixel or two. Pen strokes are narrow, and a mask that sits slightly tight will eat them.
Your signature isn’t an ordinary photo
It’s the mark you put on contracts. Every other free background remover takes that file, sends it to a server, and processes it there. Their privacy policies may well be fine, and the file has still left your machine and landed on somebody else’s.
Here the model is downloaded to your browser once and runs on your own computer. Open your developer tools, switch to the network tab, and cut out your signature: nothing goes out. It works with the wifi off, which is the plainest proof there is.
Questions
How do I make a transparent signature in Word?
Scan or photograph your signature, drop the file here, and download the PNG. In Word, use Insert → Pictures, then set Wrap Text to “In Front of Text” so you can drag it onto the signature line. The white box is gone because the PNG has no white in it.
Why does my signature have a white box around it?
Because it was saved as a JPG, or scanned straight into the document. Paper scans as white pixels, and white pixels are opaque. Removing them and saving as PNG is what makes the box disappear.
Is it safe to upload my signature to a background remover?
On other tools your signature is sent to a server. Here it is not: the whole thing runs in your browser, on your own machine, and you can watch the network tab stay empty while it works.
Does this work for Excel, PowerPoint and PDFs too?
Yes. A transparent PNG behaves the same way in all of them. In a PDF editor, place it as an image stamp.