I'm curious to know the answer to this one as well...
I tried using %vbCrLf%, chr(10) chr(13), etc. in the replace with field. I created a blank PDF and blank PDF form making sure the field value was configured a multi line to make sure it wasn't that the original searched text couldn't hold a multiple line response, but I was unable to find a resolution.
Win2K8 R2 Automate Enterprise: v11.1.1.1