You are looking for:
Scientific documents, books, journal articles, dissertations, preprints in open access?
Google Scholar:
  • searches for scientifc documents, books, journal articles, dissertations and pre-prints on university servers, publishers' servers etc.
  • displays documents as pdfs directly
  • evaluates citations: How often and by whom has a document been cited?
BASE (Bielefeld Academic Search Engine):
  • search engine developed by the Bielefeld University Library
  • offers access to about 60 million documents, such as dissertations, preprints, journal articles on more than 3.000 document servers (as of June 2014)
  • provides subject search according to the Dewey Decimal Classification
  • about BASE
OAIster:
  • was originally an open access project of the University of Michigan Library, now maintained by OCLC
  • provides free access to more than 25 millions online publications such as dissertations, preprints, journal articles from more than 1.100 institutions
  • integrated to WorldCat, but also available separately
  • contains bibliographic data and links directly to the ressources
  • about OAIster