The process of converting all pages to raster images and then OCR-ing each one takes hours for PDFs hundreds of pages long. This workflow is not suitable for instant search. For non OCR-ed PDFs it's worth to pregenerate the text.
That's why rga comes with a cache. I've occasionally used the Tesseract adapter with good success (results-wise), and after the inital rendering and indexing, it's fast enough to use.