Enter subscripts at run time

How can a user enetr subscripts into textfields when they are filling out the form?  I already have the form designed - this is not a design time question.  My users have reader 9.x and X.x  For example, they need to enter

PuP₄CO₇ into a textfield, but Acrobat Pro X or 9.x or reader/standard seems to have no font tool for the user to put in those subscripts.

Thanks,