Just to add to Brendan's comments, I think a sensible balance can be struck by turning this question around and asking yourself what you'd actually *need* to see (rather than want to see based on some arbitrary list) as part of supplier assessment and due diligence.
We don't have a particularly complex process and it doesn't contain anything proprietary, but lots of other bits of systems aren't really suitable for release - for example in addition to the usual HACCP, audit reports etc we now frequently get asked for full copies of our food defence plan, disaster recovery plan and site security plan. The latter is an interesting one as (a) much of it isn't going to mean a lot unless you know the business, and (b) releasing details of what we do about our most critical security areas rather undermines those countermeasures...
We've therefore taken a similar approach to Brendan and prepared various types of standard document that give a general overview of the system, rather than any specifics. Reception to this is about a 75/25 split between "oh, we'd never thought about the consequences of releasing details of our security systems, maybe we shouldn't do that either" and "our requirements are that we have a copy of your plan so we won't accept a statement" (it turns out that actually they will, once they've passed it on to someone with the seniority to sanction it 
).
Carine - I don't think you can completely escape the provision of information to customers, as they need to know what the product is and to assess your business as a potential supplier. Stamps etc are of very limited use these days, given how easy it is to digitally edit such things out, so I'd suggest considering a basic set of key documents for new enquiries so you're in control of the amount of info being released, e.g.:
Specification - this could be a simpler version initially, before sharing a full copy as part of proper contractual discussions. We have what we call "product information sheets" that are a short general summary of key details that we put on our website etc and by using this name it distinguishes it from a "real" spec that you can send subsequently.
Safety Data Sheet - if your products are classed as hazardous then I don't think you can reasonably avoid sharing this. There is some flexibility to write these in a way that protects confidential formulation data though (e.g. by using recipe "bandings" rather than exact amounts for components) so that could be worth looking at.
Simplified process flow diagram - lots of people are going to request a process flow, but you don't necessarily need to share the "full" one.
Certification - obviously don't share the corresponding audit reports unless you're comfortable doing so.
Allergen / GMO / Nutritional data if not included in the spec
The above would be a reasonable introduction for a "real" customer but should allow you to ensure you're not releasing too much if you're worried about spurious enquiries.
