Where and how to store this information?

Here's how you do this: 

1. You go to the detail page of your contact

2. You need to scroll all the way down until you see "Attachments"

3. Click on the green add button to add documents

4. You need to select your file(s) (for ex. business card, casual info about the customer, nice to knows,...)

5. After you have selected the file, you need to upload it

6. When you have finished uploading, you close this screen

7. The file(s)appear under attachments