A quick Windows/Mac search will find your manual much quicker than ferreting through boxes in your attic.
I don't have boxes whit manuals in my attic. Nor do I scan them for digital storage. But what I do is to keep them as much as possible in the room/environment where I may need them. This reduces the search to a minimum.
- Those of the kitchen and appliances are behind the cabin plinth
- Those of the machines and tools are in the shed.
- The printer's is underneath
- That of the TV and radio is in the box below
- And so on. and so on.