The font of your converted doc can be controlled within the advanced field settings of an Add Text or Text Field. This means that you can’t control the appearance of the font throughout the whole form, but on a field-by-field basis.
To adjust the font style and size:
Select the gear icon to the right of the field and choose More Options from the drop-down menu.
Once in the pop-up window, select Font Formatting in the upper-right corner. You will then be brought to the Field Font Formatting section where you can control the font's style and size. In this section, you can also make the font bold, italicized, underlined, or with a strikethrough, as well as control its alignment.
To view what your new font and size will look like, select Live View in the upper right corner and enter text in the field to see your changes. You will also get a preview in the Font Formatting section in Advanced Field Settings.