Permanent — there is no undo. The stored file goes with the record, unless another document
points at the same file.

Some documents cannot be deleted through the API and come back as a 409:

• a document attached to an e-signature envelope, because the envelope has to be cancelled
  in Loop first;
• a property's agency sale agreement, which the property points at by id;
• a document another record still references, such as an invoice.