So I was trying out another PDF reader just for the hell of it, and was confused about highlighting (There was a tool in the ribbon bar that I was too stupid to use).
Instead of looking through documentation, I actually tried to use the built-in AI to see what wisdom it could offer a peasant like me.
On the same subject, are there any open source PDF readers you can recommend?
Okular
https://invent.kde.org/graphics/okular
I saw this post right after looking at an MR, hehe.
And while I don't see any option to keep the highlighting tool selected, you can set handy shortcuts to almost all actions. In this case, if you have 2 hands free, you can keep a finger on the 1 key and press it to activate the highlighter right before using it with the mouse in the other hand.