Libre office draw is wonderful, it can edit pdf docs at ease. It’s crazly powerful compared to paint 3d
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
draw.io - Online diagramming application
Graphity - Graphity is a diagram editor that can be used to quickly and effectively generate drawings and to...
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.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
GeneVenn - GeneVenn is an online diagram editor that allows the users to enter the list and may use the text area in order to assign letters on the list in the diagram.