Deleting documents
In the Invoices menu, select All invoices. Choose an invoice or invoices from the visible list. Then choose from Bulk actions the Move to Bin option and Apply. Documents will be moved to Bin. You may go to the Bin to Restore documents or Delete Permanently. These actions can be done through "Bulk actions* as well.
You can also delete all visible documents in Bin by clicking the Empty Bin button.
When you delete an invoice make sure to check your invoices numbering in the Invoices menu -> Settings -> Documents because deleting invoices won't adjust the numbering.