I use paperless-ng to go mostly paperless, but I keep the paper:
I bought a simple self-incrementing stamp, which I stamp all incoming documents with. When adding the document to paperless-ng, I set the document's number accordingly, and finally I file the paper away purely sorted by ID (so I have a (physical) folder titled 1-101, one 102-231, etc.). When I need the original to a document, the lookup is very fast, and when I know I won't need the original ever again, I don't stamp it and tag it as "digital only" in paperless-ng.