Use gwenview to adjust your handwritten sign to have white color background. Use gamma, contrast. Save as separate file.
Then in command line:
$ convert handwritten.png -transparent white transparent_handwritten.png
You will now have png with transparent background.
Use KDEs Okular and add new Stamp.
You now have a stamp, that is your sign, you can put on PDF. To put sign on your document, in Okular, open “Yellow Highlighter” again and select stamp, then click / draw area where you want your sign to be.