How to backup paper wallet digitally
I wouldn't use Dropbox. Use 2 encrypted USB drives or harddisks and store both on separated locations. Storing it on 2 locations prevents it to be damaged during a fire or any other event or robbery.

The precautions to take depends on the value of your wallet ofcourse. A high value means higher precautions.

For example, you'd store $30 cash in your wallet and $1.000.000 or gold bricks in a safe or trusted bank with goverment insurance. 

If you have a $xxx or $xxxx amount, store an usb at your home and another one at your work. If the amount is higher, i'd spend some money on a safe.