Embedding a handwritten signature into a PDF document in Okular is non-obvious and broken
Problem #1: everything in the UI that talks about signatures assumes that you want a digital signature which requires a signing certificate and is super fiddly to set up even for nerds. This is probably not what most users associate with the word "signature." There is no branching path in the UI where it asks you whether you want a cryptographic signature or a dumb plain old picture of your handwritten signature embedded in the file. This is https://bugs.kde.org/show_bug.cgi?id=446549
Problem #2: Though you can in fact embed your handwritten signature into a file with Okular, it's non-obvious: you create a custom stamp annotation that has an image of your signature that you scanned or took a picture of. But this is very very undiscoverable.
Problem #3: When you do this, it looks like it worked, but the custom stamp annotation of your signature will only be visible properly in Okular; if send the PDF to someone and they open it with Adobe reader, it doesn't display properly. If you print it, it disappears. This is https://bugs.kde.org/show_bug.cgi?id=383651