When reading PDFs on my Mac, I use Skim. Although I love the program, Skim has one annoying quirk. I cannot select (or highlight) text in a split screen. This makes it difficult to highlight or download references in real-time.
I found a workaround that is fast and efficient. After opening a PDF in Skim, I’ll select “Print” from the File Menu and then, from the bottom left drop down menu, I’ll select “Open PDF in Preview.” This opens a second editable version of the manuscript side-by-side with the original.
Let me know how this works for you.