Here are some quick facts to know about documentation for an IRS tax exempt nonprofit organization:
Make your annual returns available to the public. While you don't need to go out of your way to meet someone's request, you should make these documents (your 990 forms and your application for exemption) reasonably available to the public. Be ready to provide them if someone requests them. If someone requests them in person, you should make the documents available immediately. If the request is in writing, you have 30 days to provide a copy of the information, unless it makes the information widely available.
Donor lists can be private. Generally, you aren't required to provide anyone with your donor list, even if its in your Form 990, unless you're a private foundation or a political organization.
Keep a copy of your exemption letter on file. This is important for soliciting grants, for any state or local tax issues and for showing potential donors.