How do I create relative links in a Framemaker document that is output to PDF, which links to an Excel file?

Relative to what?

An external FM hyperlink is typically either a file path or a URL.

In the trivial case, if the PDF and the XLS are and remain in the same directory - no problem. Just use the filename-only in the hypertext reference, or ./filename.xls.

A file path can be relative ../../ up so many dirs, then down some named path. The up path can change names, but if the target or any of the down path is renamed, 404. This can be sort of fixed by using a symlink/shortcut at the original location that points to the new one if the target is moved.

If the root directory is traversed (different drive letter or network object name), then the target root cannot change.

URLs pose the same problems as for any web referencing. Either things are where they are expected to be found, the server fakes it, or you are invoking some virtual object that the server has a lookup for, or finds for you.

FM, as with HTML, has a limited ability to compensate for management indecision.