Just use Postgres fulltext Search, its good enough http://rachbelaid.com/postgres-full-text-search-is-good-enou...
for postgres, i highly recommend the rum index over the core fts. rum is written by postgrespro, who also wrote core fts and json indexing in pg. https://github.com/postgrespro/rum rum handles +20mil pdf pages, interactively.
Sounds very interesting. Never heard of rum, thank you for suggesting it.