Highlighting straight line on iPad
Looks like there isn’t a way possible as iPad currently is.
But here is a suggestion on the web https://support.goodnotes.com/hc/en-us/articles/360000113556-Highlighting-with-the-Highlighter-tool
if you can find the “reconfigure tool” - I couldn’t find it.
I just open the PDF in “Books” app and it allows you to highlight it in there with a straight line.