I'm using the default Document viewer to open and highlight pdfs in Ubuntu 18.04. After I highlight something, the "Add highlight annotation" tool is automatically switched off, and I have to press its button again every time I want to highlight something. Is there any way to switch it on permanently?
SnapOverflow
It is flaky, unfortunately. There currently is no way to keep the highlight on. So indeed, life is not easy if you are reading though a document. You cannot keep your felt tip pen handy in your hand. You each time have to lay it down and close it, and you need to take and open it each and every time again for the next notation.
In Evince 3.30, life has improved somewhat, but in a different way. Evince 3.30 and upwards have an undocumented hotkey Ctrl+h that activates the highlighting, or, if you made a selection, applies highlight to it. This would work somewhat faster already. However indeed, this was only introduced in Evince 3.30, so is not yet available in your version of Evince.
Evince 3.30 also provides a "Highlight Selected Text" option when right-clicking a selection. However, this is a hit and miss. More often, the selection is not applied than applied. It works more reliably when the selection is larger, but selecting a single word, in many cases the word is not highlighted after selecting the option.
In other words, it is rather primitive in the year 2019, and we may need some more years to see the feature mature. It would be worthy filling an issue on this with the Evince developers. It makes sense having the possibility to keep a tool ready rather than to have to activate it each time over and over again.