What is the file name you downloaded? The aftermarket diagrams are rather stupid. For some reason MyMopar has now bundled up several and "zipped" them so you must "unzip" them first.
But those are not FACTORY. The service manual is what you want to start with
EDIT I see for some reason they have now "zipped" the service manuals. You must download this and save it then "unzip" it
https://www.mymopar.com/downloads/servicemanuals/1965_Plymouth_Service_Manual.zip