FreePDF: Superscripts and subscripts are possible in FreePDF only for newly inserted text boxes.
You can add a superscript (for example, r2) or a subscript (for example, H2O) to text. The commands for this can be found on the ribbon tab Home | group Character.
Select the text with the Text edit tool 
and click on the respective icon 
for a simple Subscript or Superscript.
If necessary, you can resize the superscript or subscript text by applying a different font size to the selected text on the ribbon tab Home | group Character.
Note
▪Use the command Rise if you want to make a more precise adjustment to the vertical position of superscript or subscript text.