To add an image of your signature to an otherwise finalised pdf document, first capture and store a scanned copy of the signature into an image file, such as a jpg file. Then load the pdf document into libreoffice and choose to insert an image from file. Export it as pdf.
Digital signatures are also supported by Adobe Acrobat Reader, and so probably only available with closed source software. It is problematic getting the digital signature functionality of Acrobat Reader DC to work on Ubuntu. You might need to find a Windows or MacOS computer to digitally sign a document. For example, fire up a Windows 10 Pro server on Azure, log in to your email to get the file requiring the digital signature, install Acrobat Reader DC, load the file into Reader, sign the document, email it back to yourself, and delete the server.
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