ZUGFeRD (Zentraler User Guide des Forums elektronische Rechnung Deutschland) is a German standard for electronic invoices that embeds structured XML data inside a PDF/A-3 container. The PDF provides a human-readable invoice presentation; the embedded XML (following the EN 16931 European Invoice standard) provides machine-readable data for automated processing by accounting systems. The current version is ZUGFeRD 2.x, which aligns with the European Factur-X standard. The embedded file must be attached to the PDF/A-3 document with a specific conformance level designation in the /AFRelationship entry. French implementations use the Factur-X name for the same technical specification. ZUGFeRD is required for invoices to German federal authorities (under the E-Rechnungsverordnung) and is widely adopted for B2B invoicing in Germany and France.

COVID-19 did not change the underlying legal frameworks for electronic signatures (eIDAS in the EU, ESIGN/UETA in the US) but prompted several jurisdictions to issue guidance clarifying existing laws and, in some cases, temporary regulations. In the UK, HM Land Registry issued practice guidance clarifying that electronic signatures using platforms like DocuSign are valid for property transactions under existing law. Several US states issued executive orders temporarily expanding the use of remote notarization. The EU Commission issued guidance confirming that qualified electronic signatures via remote identity verification were valid under eIDAS. Post-pandemic, most of these clarifications and temporary measures were either made permanent or incorporated into updated regulations — the net effect was a permanent expansion of accepted electronic signature use cases rather than a temporary crisis accommodation.

PDF/A-1 (2005) and PDF/A-2 (2011) are strict archiving formats that require the PDF to be self-contained — no external references, all fonts embedded, no JavaScript. The key constraint is that embedded file attachments (other than ICC profiles and XMP) are not permitted. PDF/A-3 (2012) relaxes this one constraint: it allows embedding arbitrary file types (XML, CSV, Excel spreadsheets, source files, etc.) as attachments within a PDF/A compliant container. The PDF/A-3 container itself must still meet all PDF/A-2 requirements; only the embedded attachments are unrestricted. The /AFRelationship entry in the embedded file specification must declare the relationship type (Source, Data, Alternative, Supplement, or Unspecified). PDF/A-3 is the format used for ZUGFeRD/Factur-X hybrid invoices and any other workflow where a human-readable archival PDF must carry embedded machine-readable data.