Client Document Manager

I am working for a charity and they are looking for a client document manager than is secure and safe for data protection purposes, easy to use and relatively in expensive to run. Does anyone have any experience in which ones that are on the market are the best?? It would need to be able to function with saving information from documents and surveys filled out from our website, information be easily downloaded and integrated with dropbox would be advantageous.

Any recomendations

Thanks