ubuntu - sign pdf document without printing/scanning
Using Xournal you can annotate PDFs and add custom images (e.g. a transparent PNG). Although it is used for taking freehand notes and drawing, it can also annotate PDFs.
On Ubuntu:
Install Xournal through the Ubuntu Software Center
Open Xournal
Select "Annotate PDF" from the File menu and select your PDF file to be signed.
Click the "Image" button in the toolbar (it looks like a silhouette of a person).
Click on document.
Select a PNG image of your signature.
Resize and position the image on the PDF.
Select "Export to PDF" from the File menu.
More info:
https://unix.stackexchange.com/questions/85873/how-can-i-add-a-signature-png-to-a-pdf-in-linux
Upd: you may use Signature Maker android app if you don't have PNG image of your signature yet.