Do you mean manuals for equipment which you have provided to customers?
What sort of equipment is it? What sort of manuals are they? (e.g. do the manuals have any sort of calibration data, signatures etc in them?) Do you provide any service to your customers after the sale (e.g. is there anything in the contract about maintenance, support etc?)
Generally, you'd be wanting to keep manuals etc (in one way, shape or form) for at least the statutory period of limitation (6 years) in case you end up in a dispute with your customer.
Usually your obilgations contractually are simply to supply the customer with manuals, however, where equipment is particularly high value or specialised, it's not entirely uncommon to have an additional requirement that the manufacturer is able to "make available" a copy of the manual for a set period of time (often 6 or 12 years, or longer if the equipment warrants it and the contract is negotiated in that way).
The other issue you have, aside from contractual or statutory obligations, is customer service. if your equipment lasts for 25 years on average, it would seem prudent to hang on to copies of user manuals for at least that long.
As for the method of storage, again it will depend on what the contract says, however if there is no obligation to store an original hard copy (which I would be surprised if there was, particularly if its your contract/T&Cs which form the basis of sale!), I can't see why an electronic reproduction would be an issue, even for the courts in the event of a dispute. One thing we use is the idea of a data "custodian", an independent company which will verify and securely store electronic data for 25 years. However, this is primarily so that both parties to our contracts know that the data will remain unamended.