A PDF document can be signed by inserting an image file with a signature (e.g. scanned copy of a written signature save as a .PNG file with a transparent background).
This can be done using a handwriting note-taking software with PDF annotation support that is called Xournal++.
It is an open-source and cross-platform application that supports Linux (e.g. Ubuntu, Debian, Arch, SUSE), macOS and Windows.
In this note i will show how to install the Xournal++ and use it to sign a PDF by adding an image file with a signature.
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Add Signature Image to PDF
To install the Xournal++, follow the official instructions depending on your operating system:
Once installed, the Xournal++ can be started from an application menu or by running this command from a Linux/macOS terminal:
$ xournalpp
Open a PDF file that needs to be signed (Ctrl + O):
Top Menu -> File -> Open
Select an “Image” tool (Ctrl + Shift + I):
Top Menu -> Tools -> Image
Click on a place in the PDF document where you want to paste a signature.
Locate the file with the signature (e.g. signature.png) and click on “Open” to insert it.
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The signature can be moved or resized using a “Select Rectangle” tool (Ctrl + Shift + R):
Top Menu -> Tools -> Select Rectangle
Finally save the signed file as a new PDF document (Ctrl + E):
Top Menu -> File -> Export as PDF