Admitting evidence when the authenticator can't come to trial

I have a great document I want to admit at trial, but author of the report is not coming to trial. What should I do?

Rather than try to answer this question under local jurisdiction rules, I'll try to tackle this using the Federal Rules of Evidence. For an excellent, searchable summary of the Federal Rules, see LII Rules of Federal Evidence. That should make this answer applicable to most locations. However, always check your local rules for evidence or procedure.