Glad to have you here in the Community, @snow151.

Yes, you're right. Once you unmark the box beside the attachment, your client will not see the attachments you attach.
 

Here's an article you can read for more details: Attachments in QuickBooks Online.

As always, feel free to visit our QuickBooks Community help website for QuickBooks Online if you need tips and related articles in the future.

Reach out to us if you need further help. Have a great day