This is in Sitecore 9.1 where I have a pdf file media item in sitecore, that is browsable all good from the CM site (master DB) but when I try to browse to it on the CD site (web DB), it gave me a 404 error.
When I checked the media item on the web DB I noticed that the "Media" field was empty. I went through the logs to check for any publish errors but nothing. I still do not know why this happened.