Libre office draw is wonderful, it can edit pdf docs at ease. It’s crazly powerful compared to paint 3d
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
draw.io - Online diagramming application
LaTeXDraw - LaTeXDraw is a graphical drawing editor for LaTeX.
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.
Ipe - Ipe is a drawing editor for creating figures in PDF or (encapsulated) Postscript format.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.