As far as I know, no one has fully implemented full-text search directly over Anna's Archive. Technically it’s feasible with tools like Meilisearch, Elasticsearch, or Lucene, but the main challenges are:
Converting all documents (PDFs, EPUBs, etc.) to clean plaintext.
Indexing at scale efficiently.
Managing potential legal issues.
I’ve done something like this before. Meilisearch will not be viable, because it indexes very slow and it takes up a lot of space.
In my experience only Tantivy can index this much data. Check out Lnx.
Lucene would fo fine as well, I guess. As much as I like the author of Tantivy, it is a toy compared to Lucene.