Your highlighting should be straight in any PDF editor really (not just DTTG) - you’re not actually drawing a line - as Jim says you’re selecting text and the app is colouring it. So do make sure you’re selecting the right tool (the highlighting one). If your highlighting is not straight (and assuming the text is!), it’s giving you a hint that the PDF editor isn’t treating your markups as standard annotations (which means they’re not cross-compatible with other apps, and you may not be able to extract those annotations from another app in future).
I agree with @SebMacV though - do you actually need all the bells and whistles that some PDF editors provide if you’re just reading papers (I’m a scientist and also reading a lot). I’ve been testing PDFExpert (I’m trying to fix a different issue) and I’ve ended up removing the menu for most things that offers because all I need to do is highlight, add text boxes and occasionally draw something. All the other menu options are just noise. DTTG’s PDF editor can handle all of that, as well as occasional handy extras like deleting pages, merging, etc.