How do I attach a PDF document to an invoice?

I am using the desktop app. How do I attach PDF documents to the invoice? Where can I store these PDF documents that are pertinent to a particular client?

On the client there is a documents tab, you can add documents in this location (these are public, so your client will also be able to see these)

To add attachments to email, these can be configured in Settings > Email Settings.

It says I need an Enterprise Plan to add documents in the Client section. I could toggle the switch in the Settings > Email, but I cannot attach anything under the Free plan.

That’s correct, you can see all of the plan features here: