Libre office draw is wonderful, it can edit pdf docs at ease. It’s crazly powerful compared to paint 3d
POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
draw.io - Online diagramming application
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
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.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.