From the Processing/Approvals/Review tabs use the trash-can icon beside the More button:​

In the Processing tab, if your document has more than one attachment, you can user the trash icons displayed to either delete the entire email (trash icon with an X) or delete the attachment you're currently viewing (trash icon with a U).

When an invoice has already moved to Approvals, ensure that only the unnecessary attachments are removed by using the 'Delete single file' option. Avoid using the 'Delete entire email' option in this scenario, as it could inadvertently remove the processed invoice.

From the Signed Off tab, you need to return the bill to the previous tab before trashing it.
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Anything that has been trashed is retrievable for a limited time of 30 days.
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You can retrieve a bill from the Trash for up to 30 days by searching for it in the Archive. Just make sure that "Include Trash" is selected in the filter options.

If you have trashed a document from the workflow, they have been extracted and therefore linked to supplier/document details. However, if you Trash an item from the Processing tab, it will not have the defined data linked to it. Therefore, you will need to search for Trashed Processing from the dropdown list. ​

If you've accidentally deleted a bill, don't panic! Now you know how to find them, you can search for your trashed items, select the document you're after and click Reinstate bill to bring it out of the Trash and back into the Processing/Approvals tab. Presto! It's back in your workflow.