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
EvolveUI - Tribaloid is developing a screen design tool code named UnderDog specifically tailored for user experience designers, information architects, interaction designers, and web developers.
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.
CorelDRAW Graphics Suite - Empower your creativity with CorelDRAW Graphics Suite 2018, packed with powerful features and updates to transform creative ideas into visually stunning designs! Learn more or download a free trial today!
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.