Notability has two different highlighters: Smart Highlighter and Straight Line Highlighter. 

Smart Highlighter

Our Smart Highlighter feature allows for faster highlighting.  

Long press on PDF text to snap and begin highlighting.

Make sure you are long pressing the first word of the sentence you want to highlight before dragging and continuing to highlight.

Highlights can be edited, styled and erased like regular ink.

Straight Line Highlighter

Draw straight line highlights by drawing a line over text, and then holding.