Libre office draw is wonderful, it can edit pdf docs at ease. It’s crazly powerful compared to paint 3d
open-pdf-sign - Sign PDF files digitally from CLI or your webserver
draw.io - Online diagramming application
TinyWow - TinyWow provides free online conversion, pdf, and other handy tools to help you solve problems of all types.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
Wondershare PDFelement - All-in-one PDF editor
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.