This post si a demo of the PDF Embedder plugin.
It contains a tag linking to a PDF document url:
[Document inserted with PDF embedder shortcode for a demo
pdf-embedder url="https://www.wpsolr.com/wp-content/uploads/2017/01/Premiers-pas.pdf" title="Document inserted with PDF embedder shortcode for a demo"]
Each time the post is saved, the PDF document is downloaded, it’s content extracted and inserted in the post content, during Solr indexing. See details.
Then, if you search for a keyword contained in the PDF document, this demo post will be displayed in the results, and the keyword will be highlighted. Exactly as if the PDF was copied in the post content.