Mac Preview - help

[Katiedid]

I use Preview on Mac to mark up my crossword grids (PDF) after I do them on acrosslite. I had not updated my OSX in a long time (don't judge me), so on my old computer I could drag to select one entry and highlight it. But now I have a new MacBook Air with the latest OSX, and when I try to do that it selects all the letters in columns when I drag across a row. So I have to click each individual letter! Does anyone know what to do about this? This is what it looks like when I was just trying to highlight SCARLET.

[MMe]

I use Skitch to capture a screenshot of just the grid. Skitch has a tool to highlight in different colors.