Data protection Solution

I have a new startup idea where I will serve clients in the financial services space. We want a platform/tool/solution where my client will upload (Excle/word) files on a particular platform and my team can work on it without being able to able to download or save files on the local PC/server. The objective of the practice is to gain clients' confidence in the data security/protection aspect. Please suggest me way to do it. Please note that you just need to suggest me and not perform any task for it.