Six document search engines you should use
Its creator, Dr. Michael Bell, explains “As a meta-search engine, the Academic Index integrates into its search results only the first 1-2 pages returned from each site it searches. Because most sites rank search results as to relevance, this ensures that only the best (most relevant) information is returned to users.” 
The Bielefeld Academic Search Engine searches for academic web resources: journals, institutional repositories, digital collections etc.
The Directory of open-access journals gathers documentation on science, technology, medicine, social science and humanities (approximately 10.000 journals). The aim of the DOAJ is to increase the visibility and ease of use of the journals to promote their use and impact.
DocHound is the EU Interinstitutional Document Search tool by the Terminology Coordination Unit (TermCoord) of the European Parliament and it updates its content regularly, so you are sure to get up-to-date documentation. You will find basic documents, legislative drafting, procedures, documents from the EP and other institutions and bodies.
- CORE (COnnecting REpositoires)
CORE gathers content from repositories and journals around the world. CORE harvests all metadata records in a repository. For now, they only offer PDF files but hope to expand the service to include HTML, webpages, etc.
This great site is like the Google for academics, science, and research. It strips results to show pages such as .edu or .org and includes more than 1 billion publications, such as web pages, books, encyclopedias, journals, and newspapers. In a test done by IT journalist, Stan Schroeder, when he searched for “flower”, RefSeek showed him documents from botany (as compared to Google that returned a list of florists!) 
For a comprehensive list and by topic, I recommend checking these pages.
- Top 11 Trusted (And Free) Search Engines for Scientific and Academic Research
- 100 Time-Saving Search Engines for Serious Scholars (Revised)
Share your favorite engine in the comments or send me a note to add it here.
References: Schroeder, Stan. RefSeek is Google for Students and Scientists. 2008 [consulted on 2/1/2018].  Bell, Michael. Academic Index. 2003 [consulted on 2/1/2018].