Dissertation and Theses - Full Text (ProQuest) contains entries with information about doctoral dissertations and master's theses. It is the same database as Dissertation Abstracts, but with the significant advantage that titles published since 1997 are available in PDF digital format and have 24 page previews available.
If there is no option to download the full text of a post-1997 dissertation, this means that the author has refused to grant these rights.