ElasticSearch allows extending its core types to cover new requirements with native plugins that provide new mapping types. A most used custom field type is the attachment one.
It allows indexing and searching the contents of common documental files, that is, Microsoft office formats, open document formats, PDF, ePub, and many others.
You need a working ElasticSearch cluster with the attachment plugin (https://github.com/elasticsearch/elasticsearch-mapper-attachments) installed.
It can be installed from the command line with the following command:
bin/plugin -install elasticsearch/elasticsearch-mapper-attachments/1.9.0
The plugin version is related to the current ElasticSearch version. Check the GitHub page for further details.