How long documents are kept, and what can be deleted

Drafts have no number yet. You can delete a draft at any time with "Delete draft" in the editor.
Finalizing gives the document its number and stores its PDF under a compliance lock. From then on nobody can delete it, an admin neither.
Invoices are kept for 8 years from the end of the calendar year the document was issued in (§147 Abs. 3 and 4 AO, §14b UStG).
Quotations, order confirmations and delivery notes are kept for 6 years from the end of the calendar year the document was issued in (§257 Abs. 1 and 4 HGB).
A wrong invoice is never deleted. If it was not sent and nothing was paid on it, you void it. Otherwise you issue a cancellation invoice (Stornorechnung) or a corrective invoice (Rechnungskorrektur).