How do you change the lettering style of a adobe document that has been sent to you?
If by a "adobe document" you mean a PDF file, and you only have the free Adobe Reader, the answer is simple: you can't.
You can do this with Acrobat, but, as Claudio says, it's not possible in Adobe Reader.
If you do have Adobe Acrobat, you can learn how to modify text styles here.
Europe, Middle East and Africa