Libre office draw is wonderful, it can edit pdf docs at ease. It’s crazly powerful compared to paint 3d
draw.io - Online diagramming application
Invision - Prototyping and collaboration for design teams
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.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Axure RP - The prototyping and specification software used by 87% of the Fortune 100. Beyond mockups, design interactive prototypes for business solutions, websites, and applications.