What controls the list of fields that are displayed in the dropdown list for setting the conditional start criteria?

The workflow is a site collection reusable workflow associated with a Content Type. I assume that child Content Types will automatically be associated also? 

I have tried to attach the built-in field Version as an associated site column to the workflow. That did not include the field in the dropdown list. 

Then I included the field in the Content Type, which did include the field in the dropdown list. However, this have some other side effects , such a showing the field in the side panel on the list, which I do not wish. 

Basically what I am trying the achieve is to trigger only for a specific file type, for instance PDF and only on major versions. So other suggestions to achieve this is also welcome.