There are a few things you can do:
When you are on the "Design" tab of your form go to "File" - "Form Setup". In Form Setup you can define the form width to something that is mobile friendly, since there are so many variations in mobile you will have to figure out what pixel width works best for you (for an iPad 1000 works well).
Another thing you can do is only add a few fields per page so most or all of the form fits into the page and the user clicks Next to go through the pages/fields.
Larger fonts, like 16 will look better on mobile as well.