How do I allow customers to view invoice attachments? (QuickBooks Online)

If you attach files to a QuickBooks Online invoice, your customer can view these attachments when opening their invoice in the portal.

Can customers view ALL invoice attachments?

It depends. In the settings page, you can click to enable or disable whether to only show attachments that have been marked "Attach to Email". If you check that option, your customers will only see attachments you have flagged for being attached to emails. GoToMyAccounts assumes that attachments to be sent to your customer should be view-able in their portal.