Took the nudges to discern the mechanism, though I had the thought all along. Still I needed to enter the black squares on a blank Excel 15x15 template and add the "extra" squares to create symmetry to identify the answer squares/letters, as I couldn't get the artwork to work on my PDF copy. Must have been a challenge to create the initial grid.
And now I see that there is an Excel sheet among the puzzle format choices. Using that early on would have made my life a lot easier. The PDF really obscured matters.