Keynote 8.2 vs. Keynote 6.6.2
We have multiple keynote files created in High Sierra with Keynote 8.2 which open ok in Keynote 6.6.2 in El Capitan. However, if in El Capitan we save the file after even a minor change, the new file cannot be re-opened in El Capitan and requires us to open a prior version. This is very repeatable on multiple computers. Does anyone know the 'feature' of 8.2 which is causing this? BTW, if I export the file as a Keynote 09 file and import it into Keynote 6.6.2, change it and save that file, it seems to work fine. This suggests some 'new' incompatible 'feature' not being correctly interpreted. Any idea what to fix?
Mac Pro, macOS High Sierra (10.13.6)