Any file that has been shared with you on Witkit can be found in Witkit Drive, in the folder 'Shared With Me.' There are 2 ways to download a file from Witkit Drive.
Option 1: Click on the More icon and 'Download'
Option 2: Click on the file and then use the Download icon
Just a note, you do not need to download a file from Witkit Drive in order to edit it.