I was just on Simon & Schuster's Canadian web page and I just noticed that the 1997-2000 Frank and Joe Hardy Are: The Clues Brothers/The Hardy Boys Are: The Clues Brothers has had all 17 titles available as ebooks since 2013. I couldn't find any mention on the site about the Clues Brothers ebooks, looks like they released all 17 books between August and October 2013.
So for any Hardy Boys fan with young kids in the Secret Files/Clue Book age range, you can get this series legally on any ebook platform that allows your kids to read this series from the late-90's/very-early-2000's.
Good to know. I honestly have found this series to be better than the current Hardy Boys series.