Need details and help in understanding DBpedia API

Let me take this opportunity to explain our product called J-Gate which is a bibliographic database acting as a discovery product for Scholars, Researchers, Scientist community and Post graduate students.

J-Gate indexes 52000+ journals along with thousands of Theses, Conferences etc. We have clients scattered with category from Aeronautics to Zoology.

Since ours is a bibiliographic & scholarly database , we need to provide powerful search tools for narrowing results, where users should be able to more quickly find the information, they need.

We are looking for a solution in annotating the documents to generate keywords for entire documents of J-Gate( ~70 million plus) in bulk.

How does DBpedia helps us in achieving this?

What are the skills required to use the API available in DBpedia?,

How to use the Open knowledge graph of DBpedia?

Do you have any services offered for our kind of requirement?

Can you share the ontology being used for annotating the document in DBpedia? for our study purpose?