I haven't tried it, but I suspect it "might" work with some documents but fail with others because the FTP stages in DataStage have historically been "streaming" mechanisms. You need to send a "whole" document into XMLInput....if in streaming, the FTP Stage "thinks" it has a single record (if it sees what it things is an end-of-row indicator), it will send it. If that's not the entire file, then XMLInput will fail. Perhaps someone with a lot more experience with the FTP ENterprise Stage can chime in....it's the behavior of that Stage that will have a large impact here.
Use a Before Job Subroutine and call and automated FTP or SCP script to copy to a local folder, then use External Source to cat the file contents to its output link and pass this output link to XML input