NAPS2 (Not Another PDF Scanner.) Free versions for Linux, Windows, and Mac are available for download. Simple interface but fully featured, NAPS2 is much better than the overly bloated proprietary software that comes with most document scanners. Compatible with many devices.
I think I phrased this badly. I wanted effectively a digital filing cabinet software with OCR/tagging—not just OCR alone. It looks like paperless-ngx ticks all those boxes.