Some things belong on a client's record but aren't words: the consultation form they filled in on their first visit, a signed consent, a photo of a patch test. Words go in Notes; files go here.
Filing a document
Open the customer in Customers and choose the Files tab. Drag files straight onto the page, or click to browse - several at once is fine. A file can be a PDF, a Word document, or a photo straight off your phone (snapping the paper form is the quickest way to file it). Each file can be up to 15MB, shows who filed it and when, and can be renamed - handy for turning "IMG_2043" into "Consultation form, first visit". The file type never changes with the name.
Who can see them
Your team only. Files are private business records: the customer never sees them, they're never part of the booking page or any email, and links to open them expire within minutes. People on your team who can open the client book can read files; anyone who can take bookings can file and delete them.
Plans
Keeping files on customers is part of the paid plan. If your plan changes, everything already filed stays exactly where it is - you can always open and download your records; only adding new files waits for an upgrade.
Deleting one
Press the bin on the file's row, then Delete file. It can't be undone, so it's for things filed by mistake, not housekeeping.