It is not the pdf file. What is unusual is I have .est installed on a desktop and then a company server. The server file will not import underlays, however my desktop imports them just fine. My workaround is to create the est file in my desktop est and put in all the underlays I need and then copy and paste the file into the server and open it in the server. The underlay shows up just fine then. ( I can't use the desktop est for other reasons)

The only difference is the company server doesn't have the advanced acrobat reader software like my desktop does, but I don't think that should make a difference.