That's really nice, thanks. Long ago there was the Google Desktop Search where you could 'Google' your local documents. But the difference is that that worked with an index, so I imagine it's faster if you have thousands of pdfs en epubs.
Even longer ago, there was `glimpse`: https://www.linuxjournal.com/article/1164 which is still available. [1] glimpse's index-builds are like 10X slower than `qgrep` mentioned elsethread. `qgrep` also seems to have faster search (though I only tried a few patterns) and `qgrep` does not allow spelling errors like `glimpse`.
Neither `glimpse` nor `qgrep`, to my knowledge, directly supports pre-processing / document conversion (like `pdftotext`), though I imagine this would be easy to add to either replicating Desktop Search. (Indirectly, at some space cost, you could always dump conversions into a shadow file hierarchy, index that, and then translate path names.)
