How to Sign a Document with GIMP

Someone asked you to sign a doc and email it back to them, so now you’ll have to buy a printer and scanner, print the doc, sign it, and scan it. … NO! Stop! 

Why waste paper when it’s been possible for ages to sign your documents digitally? All you need is an image of your signature, the PDF you want to sign, and GIMP! Let’s see how!

The Prerequisites

To be able to sign a document digitally, you will need the document in PDF format.

Adding your signature to the document is easy if you have a pen tablet. For the majority of people who don’t have such gear, a photo of your signature snapped with your smartphone and saved in JPEG format will do. 

There is plenty of software that allows you to sign your document. In this tutorial we will show you how to add your signature using GIMP.

Related: How to Draw Arrows in GIMP

Getting Started

Run GIMP and use “File -> Openv…” to find, select, and open the PDF you want to sign.

The PDF in the example has a single page. If yours has more, select that you want to “Open pages as: Layers.” Leave the rest of the options as they are and click “Import.”

If your doc consists of more than one page, thanks to the previously selected option, you will find them as individual pages in the layer panel on the right. Use the little “eye” icon on the left of each layer to toggle its visibility.

Use “File -> Open as Layers …” again to find, select, and import your signature image as a new layer.

Playing with Layers

Move the newly imported layer of your signature directly above the layer of the page you want to sign by left-clicking and dragging it up or down in the layer list. You will have to click and un-click some “eye” icons to find the proper one.

To make its placement easier, reduce the opacity of your signature layer using the bar on top of the layer panel. Use the “Move” tool (“M” on the keyboard) to move your signature layer where you want it to appear on the page.

Use the Scale tool (Shift + S on the keyboard) if you need to scale down your signature for a better fit. After you select the tool and start scaling down your signature, press Shift again, and keep it pressed to constrain the transformation proportions. You wouldn’t want your signature distorted.

You might need to select the “Move” tool again to better reposition your signature on the page. When you’re happy with its placement, bring the layer’s opacity back to 100%.

And now, time for our magic trick! Others would tell you that you need a transparent PNG of your signature. You don’t if your signature is, as in 99.9% of the cases, black or blue ink on white paper. And we have layer modes to thank for that.

Related: How to Resize and Optimise Images Using GIMP

Notice the “Mode” pulldown menu on the top of the layer panel. With your signature layer selected, change its mode from “Normal” to “Darken only.” And there you go: all white space around your signature will be ignored, effectively turned transparent, with your signature “darkening” the layer directly underneath. (That’s what the mode we just selected does.)

And that was it! To finalize the changes, right-click on your signature layer and select “Merge Down” to combine your signature with the page.

Select “File -> Export As …” and give your newly signed document a different name to keep the original intact. Use “PDF” as its extension to export again in this format.

The reason we told you to “Merge Down” your signature to the page it affected is because now GIMP will export all layers as pages of a new PDF. If you had kept your signature on its own layer, it would have been exported on its own page instead of being “burnt in” where you wanted.

Apart from being very useful and very simple to do, what we saw also works as proof that GIMP can do almost anything, including saving files to webp or creating a GIF from a video file.