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
TexitEasy - TexitEasy is a free, cross-platform and open-source latex editor.
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.
latex4technics - Online LaTeX editor with autocompletion, highlighting and 400 math symbols.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Lagrida Latexeditor - Lagrida Latexeditor is an all-in-one website that lets you edit, write or check the authentication of your mathematical equations or symbols in a decent manner without any hassle.