To make mathematical annotations on the computer I use Notepad, but Notepad has neither a formula editor nor almost any editor, so it very quickly starts to get harder to read equations. The other pieces of software I have seen were robust, as Evernote. For Notepad, I press two buttons, type “notepad”, press enter, and I have a simple and fast notepad, whose files I can save easily, differently of Evernote. Just a Notepad with formula editor would be great...
Hence, my question: is there any software good for annotating stuff? By the way, I know it is just extreme convenience and even LaTeX would suffice.
If $\LaTeX$ will suffice and you are willing to learn it, use it. You can learn most of the math syntax right here on this site from a mathjax tutorial. Texlive is a good application to download. Overleaf does TeX in the cloud for you.