Libre office draw is wonderful, it can edit pdf docs at ease. It’s crazly powerful compared to paint 3d
Signavio - The Signavio Process Editor was created in order to be very intuitive and easy to use.
draw.io - Online diagramming application
Miro - Scalable, secure, cross-device and enterprise-ready team collaboration tool for distributed teams. Join 2M+ users & 8000+ teams from around the world.
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.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.