In order to attach a file, you will need to have already created an activity. You can create an activity in a variety of places: Leads, Orders, Schedule, Accounts, and even the Activity List.
Find the activity you want to attach a file to from the account page or activity list.
Click on the “Files” tab.
Here you can click on the “Choose Files” button to upload a file.
The file will now be included with the activity, and you can now access it from any computer.