As said in another comment, I am very happy with Xournal++. It let you add pictures (I draw my signature once in Photopea and saved the file) and text (useful to fill forms).
In short, Xournal (original, not ++) can add pictures and text too! And it's available via official Debian repository. So it's easier to install in Debian.
Xournal++ is a rewrite of Xournal, and has more features, but I don't want to deal with downloading a .deb or using snap or flatpak. They're working on getting Xournal++ into official Debian repository though, but it's not there yet.
Xournal (original, not ++) is quite good for filling in basic forms on Debian!