Entrepreneurs or professionals are obliged to keep the following documents with their original content and in an orderly fashion during the limitation period provided for in the LGT:
1. Invoices received.
2. Copies or matrices of invoices issued.
3. Invoices and accounting receipts issued in cases where goods or services are supplied to the recipient, who is the taxable person liable for the tax.
4. With regard to the REAGP for VAT, the receipts, both the original by the issuer and the copy by the holder of the holding.
5. In the case of imports, the documents mentioned in LIVA art.97.uno.3º.
They are also obliged to keep the aforementioned documentation:
- entrepreneurs or professionals covered by the special VAT regimes;
- those who, not being entrepreneurs or professionals, are taxable persons, although in this case it only applies to the documents mentioned in points 1 and 3 above.
The preservation obligation may be physically carried out by a third party, who will in any case act in the name and on behalf of the entrepreneur or professional or taxable person.
Prior notification to the AEAT is required if the third party is not established in the EU, unless it is established in the Canary Islands, Ceuta or Melilla or in a country with which there is a legal instrument of mutual assistance with a similar scope of application to that provided for by Dir 2010/24/EU and by Rgto UE/904/2010.
Up to now, the tax authorities have been going round in circles when asked whether sending a PDF invoice by email was valid. It simply stated that the sending system had to guarantee the authenticity of the origin and content of the invoice, and that recognised systems (e.g. advanced electronic signature) or management controls could be used to create what is known as a "reliable audit trail".

In PDF and by e-mail? You can do it!

Well, although with these answers it could already be understood that sending an invoice in PDF format was valid, now the Tax Authorities have gone further, and have confirmed the validity of this form of sending, even if it is done without an electronic signature (DGT V0553-15, V0340-15 and V2426-14).
Audit trail. However, it is still essential to have correct management controls and an audit trail. However, this will not be difficult for your company:
A correct control system is one that verifies the connection of the invoice with the purchase order, the delivery note and the payment order.
And an audit trail will be reliable if the purchase order, the transport documents and the invoice are kept together.