I suggested that, but they insist they need a "handwritten signature".
Honest answer: Get yourself Adobe Acrobat Reader (Win only). Even the free version allows you to do add a signature be it free-hand sketch, or as an image (they also have a cloud version of that). It's a pain, but does the job. Also,you can generate a SMIME certificate and add it into the document. Again, Adobe-specific solution. It's the only reason I keep Windows VirtualBox VM around.
If you don't want to go Adobe way, another option is to use Xournal to open the PDF in question and scribble your stuff onto it. Not great, but again, does the job. Sort of... Finally, there's LIbreOffice Draw which opens any PDF and allows you to edit it and scribble into it. The only problem is that when you save it back to PDF, sometimes it screws up fonts and there's little you can do about it. So YMMV. Optionally, there are some SaaS options for this out there, but I did not try (except the original Adobe thing).
Oh, and there is also Master PDF Editor which could help. I think the life-long license is worth avoiding the Adobe crap. Free version adds some watermarks, but if you are OK with that, you might be good.
Interesting. This looks like going Inkscape way. It's a vector editor. The problem with Inkscape for this is that it cannot properly open and edit multi-page PDFs. Does this Karbon thing do it?
(before I try I better know, becuase that would require me to pull in GBs of KDE dependencies first, I guess)
