It was painful to watch. I often sign PDF documents on Linux. I see three ways of doing it:
- using annotations in the PDF viewer. Okular supports this, I think Evince too. So you don't have to install anything normally
- using Xournal. That's was I do.
- Pasting an image of your signature using LibreOffice Draw. I know someone doing this.
It's pretty simple and you don't need to install Wine + a Windows software to do this. But you have to know it I guess. I'm sure I would be in a similar situation as them, should I have to sign a PDF document on macOS or Windows. I know the PDF viewer on macOS does that from observing former colleagues. On Windows I would just install Xournal I guess.
There could be a "Sign this PDF" button in PDF viewers to make this common action obvious and solve this particular use case.