How to fix PDF-20
A font is not embedded
A check of the PDF of a ZUGFeRD or Factur-X invoice, not of its XML. It runs when you upload a PDF; the embedded XML then gets the XSD and EN 16931 checks (XRechnung: also the KoSIT rules). Check your PDF with the ZUGFeRD validator or the Factur-X validator. What the PDF check covers.
PDF/A-3 (basic check). This is one of a few basic PDF/A checks. They are not a full PDF/A validation: rules such as CIDSet, transparency or tagged structure are not checked. For a full PDF/A-3 check, run the PDF through veraPDF.
What this rule checks
PDF/A requires every font used to show text to be embedded (FontFile, FontFile2 or FontFile3 in its font descriptor), so the invoice looks the same everywhere. This checks the fonts in the resources of the pages and their form XObjects; Type 3 fonts need no embedding.
How to fix it
Embed all fonts when creating the PDF (for PDF/A export most tools do this; standard fonts such as Helvetica must be embedded too).
Related rules
Validate your invoice
Upload your corrected invoice to check it passes PDF-20 and all other ZUGFeRD / Factur-X PDF checks and EN 16931 rules. Free, instant, no signup.