What are the applications of Solr in Big Data?
Solr is a highly scalable, fault tolerant search server.

So, you can store files in the form of JSON, XML, CSV or binary over HTTP.
And you can query it via GET and receive JSON, XML, CSV and binary results. It also has a distributed architecture for the purpose, which is the primary and most important concept in Big Data and it's handling.

So, as it is highly scalable and fault tolerant for high traffic and huge documents, it is a choice for a reliable search engine for huge documents, or in short Big Data.