It is just one of those things you have to bite the bullet and do. However, if you feel the line is being crossed talk with your manger and upper management to let them know you don't want to share the information. Sometimes, just a discussion over the phone can clear up a lot of things with your customers.
Personally, I'm in the process of putting together a general facility quality & food safety overview with various documents as a one PDF file and a similar type product group specific packet for customers. I get these from some of our major suppliers and it provides a lot more information than I would ever need or ask for, and all in one packet.
I've thought about putting together a packet. We have a couple of suppliers that put together a packet in a .zip file for our documentation needs; and usually everything we need is there. However, as suppliers ourselves, we still get the occasional request for random other documents that we've never seen or heard of before. So we usually just stick to the old attaching each document to the e-mail method.
I do find that there are some requests that we do not complete; especially related to TonyM's original question. We provide a very detailed HACCP Flow Diagram and will answer questions related to CCPs, but there are some parts of the process that we're not willing to distribute. We are co-manufacturers to several organizations; and some of them are very specific as to not divulging certain parts of the process. Our HACCP plans are allowed to be viewed on-site, but we do not distribute the majority of the content. Our customers have been fine with that.
As previously stated by other posters, a meeting or a conference call may need to be considered between the company and the customer to discuss how to make it suitable for both parties. Unfortunately, that could mean making additional documents. However, that may be the best option in this case.