I completely understand. I am a 100% cloud based practice. I utilize QuickBooks Online and Proconnect Online Tax. Both of these environments allow me to attach my supporting documents directly to the transaction. In the event there is a request for any information I can navigate to the item effortlessly.
I am also a huge fan of LedgerSync which allows me to retrieve the bank statements and checks of each of my clients.
However, for my engagement letters and other documentation I utilize Google suite: drive, powerpoint, excel, and word all at my fingertips. Since I change my passwords so frequently I feel confident that my documents are secured AND I can access from anywhere.