Hi Paul,
The simple answer here is that DSpace can support any file format if you just want to upload the file and get the same file back (when a user downloads it).
But, you are correct that DSpace doesn't have any specialized TEI support. So, there's no TEI viewer and DSpace won't index the TEI file by default, so you won't be able to search within the text of the document. That said, it's
possible that the out-of-the-box HTMLFilter (
https://github.com/DSpace/DSpace/blob/dspace-6_x/dspace/config/dspace.cfg#L372) might be able to parse text out
of a TEI XML file if you configured it to do so (it may not be perfect though as XML and HTML are only loosely similar in structure).
In any case, if you upload a TEI file into DSpace, then users will be able to download that same file. Obviously, you can also add whatever metadata to it you want to make it more easily findable. As you noted, you could also convert them to PDF or upload
both the TEI and PDF versions into the same item -- it may depend on your local needs/use cases as to whether downloading a TEI file is best for your users or if another format is also useful.
Tim