You are more than welcome to use different logos for each of your clinics if you want, but you don't have to! By adding a "default" logo, you can ensure that the same logo is used across all of your PDF documents (invoices, payments, account statements, treatment notes, and letters).
Upload a default logo
Head to Settings, and then Documents & printing:
Click the Choose file button:
Select your logo from its spot on your computer, and upload it. You won't see it right away—first, click the Save document settings button at the bottom of the page:
Head back into your Documents & printing settings, and you will now see that your logo has been uploaded:
This will now be the logo that is used across the board on all of your PDFs, unless you opted to upload a unique logo for each business. To ensure that the logo displays on the correct documents, you'll also want to be sure that you follow the steps for getting it to display properly. Those can be found here:
The process is slightly different for online bookings, though! If you want to show a logo on your online bookings page, head over here for the steps.