There is not a direct way to do a drop shadow. There are a few options that may work for you:
There are some fonts that include that feature automatically. You
could upload one of these fonts to the template and use that template
for the field in question. This will work in both standard forms based and interactive designer templates.
b) On a standard eDoc template, you can
have two separate fields. Set the fields almost exactly on top of one
another, but the field below would be just very slightly offset from the
field on top. On the "Field Type" tab you would set the field to be a
text field coming from another field. This will work in a standard forms based template.