Global Find/Replace with Subscript

I currently have a situation where a PDF is being generated with H2S, CO2, etc. for chemicals.  I am unable to change the base system to support subscripting the numeric in the chemical symbol.  These PDFs have many of them.  Frequently is the some one or two throughou the PDF like "H2S".  I am trying to use the Find/Replace feature to replace H2S with

H₂S (as you can see the "2" is subscripted).  However, when I cut and paste H₂S into the replace field, the text in the document still shows as H2S.  Even if I change it on a single entry by adding the subscript and then copy and paste the modified entry in the "Replace" field it does not work.

Any idea how to do a global find and replace on H2S to H₂S?