(Linux) Load the PDF in xournal, click on > Tools > Image. Select a jpeg holding my signature. Change the dimensions and drag the signature around as needed. Note that you then have to export to PDF rather than saving it.
(Android) Using the OneDrive app. There's a signature option in the annotate menu.
Xournal can be installed from the official Debian repository, but not Xournal++ (not yet - it's apparently being worked on).
So Xournal is very convenient for filling in basic forms on Debian and is easier to install.
Xournal++ could be installed on Debian by downloading a .deb, or using snap or flatpak. But not as convenient as installing Xournal via apt-get.
When Xournal++ makes its way into the official Debian repository, I might switch to it.
For now, I'm very happy with Xournal (the original)!