There are different approaches to distinguish e-invoice formats.
Not every electronic invoice document qualifies as an e-invoice in the sense of the European standard EN16931. They have to be XML documents using one of the syntaxes UBL or CII.
What is meant with syntax? A UBL and a CII invoice can contain the exact same information. What differs is the name of the XML elements and their location in the document object model.
Both UBL and CII allow embedding a human-readable, normally PDF version of the invoice to be embedded in the XML.
In France and Germany, the standards Factur-X and ZUGFeRD for e-invoices have been developed. Both Factur-X and ZUGFeRD documents are actually PDF invoice documents but they contain an equivalent XML version of the invoice as an attachment to the PDF.
Factur-X and ZUGFeRD are nowadays identical standards. They are just different names for the same thing. Factur-X is the official name but the term ZUGFeRD is still prevalent in Germany.
An electronic invoice is either an XML document in one of the flavours UBL and CII or a Factur-X/ZUGFeRD PDF. If you opt for an XML version you may include a PDF version. If you go with the hybrid format Factur-X/ZUGFeRD you must include an XML version of the invoice.