Why files get lost
One document sent by email, another saved locally, another in a teammate's personal Drive, each separate location is one more place you can search without finding it.
The simple rule that helps
Every file relevant to a client belongs to that client's documents, not someone's personal inbox. When the rule is clear and followed, searching becomes "open the client", not "ask everyone".
What you actually gain
Less time searching, fewer "can you resend that?" messages to clients (which leave a bad impression about your organization), and an easier handover if someone on the team leaves.
Bottom line
Organizing documents isn't a big archiving project, it's a simple rule applied consistently from day one: every file has exactly one place where it lives.