How do I set tab order in fillable forms [2017]

In the "Prepare Forms" tool the fields are listed in  the tab order under the heading "Fields". You can drag and drop the fields to change the individual tab order. The first icon to the right of the fields heading is a drop down list that lets you macro order the tabbing and the second icon (A-Z) lets one select the order in which to display the field names.

In recent versions of Acrobat, you go into form editing mode (e.g., Tools > Prepare Form) and in the fields list you can drag field up/down to set the tab order. At the top of the fields list there is a dropdown where you can specify by row, column, structure, etc.