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
SuperNotecard - Introducing SuperNotecard. SuperNotecard is an online writing tool that features virtual notecards to help arrange facts or scenes, track details, organize paragraphs, and clarify your composition process.
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.
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Freeplane - Freeplane is a powerful and free software for building mind maps.