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We're a small government research company with hundreds of clients. We are using Linux and Apache for our public web servers, all hosted in a remote data center. We have a massive repository (hundreds of GBs) of research documents (PDF, Word, Excel) that are available to our customers on a subscription basis -- once they've paid for a subscription they can login to our web application and access any of these hyperlinked documents, which are currently just sitting on our web server. These documents do not have any sort of protection on them, and the hyperlinks can be shared with anybody. There is also no versioning of the documents -- when a document is updated, there's no way to track what was changed. Can you give us an idea of the best way to protect our document repository, without adding frustration for the current customers? In addition, how can we track the versions of the documents and allow customers to see what was changed? We are assuming that we need some sort of document management system, but we need advice on which one would serve our needs, and our budget.