Based on your documentation, it seems that WP File Download reads in the contents of the files you upload. If that is indeed the case, where the content is stored in the WP database? I'm the author of the Relevanssi search engine for WordPress and have users that use WP File Download and would like to have Relevanssi access the PDF contents in WP File Download -managed files. Is the content stored in a custom fields or database tables?
How is the attachment content reading implemented? Is it done locally on the server, or somewhere else?