In contrast to RTF, RTFD files cannot be opened properly by applications on non-Mac OS X operating systems, as bundles are currently only supported on Mac OS X.
While there might not be any GNU/Linux applications that support.rtfd files, the d in .rtfd stands for directory. That is, this is actually a folder, and you can look inside it to see the constituent files which provide the document's content. This will often make it possible to access the document.
Also, that article suggests that Ink.app, and possibly other GNUstep programs, can open RTFD files; it should be possible to install that in Ubuntu, though I don't think there's any official Ubuntu package that provides it.
As far as I know, it is a mac file and it can't be opened in Linux.
See also in Wikipedia.
While there might not be any GNU/Linux applications that support
.rtfdfiles, thedin.rtfdstands for directory. That is, this is actually a folder, and you can look inside it to see the constituent files which provide the document's content. This will often make it possible to access the document.See this Wikipedia article for more information about the RTFD format.
Also, that article suggests that Ink.app, and possibly other GNUstep programs, can open RTFD files; it should be possible to install that in Ubuntu, though I don't think there's any official Ubuntu package that provides it.