I would suggest you save the clients details as a Quick Part that you can just insert each time. A drop down list wouldn't work for that much information.

Type in the clients details (name & address). Highlight the details and go to the Insert tab and click the Quick Parts button and select Save Selection to Quick Part Gallery. Change the name to that of your client and click OK.

Next time you go to create an invoice for them, place your cursor in the area the name & address would go, go to Insert tab and click Quick Part button and choose the client from the list of choices and it will insert the text for you.

More info can be found at https://support.office.com/en-us/art...1-aae0774e4813