PDF/A

PDF/A is the PDF of long-term archiving: a document that shows the same in decades, without anything external (fonts, color profiles) and without encryption or scripts.

// PDF/A embeds the fonts: the standard 14 fonts cannot be used
var font = PdfFont.Load(fontData);
var document = new PdfDocument { PdfAConformance = PdfAConformance.PdfA2B };
document.Information.Title = "Archived";
document.Pages.Add(PdfSize.A4).CreateContentBuilder().DrawText("Office", font, 24, 60, 700);

// The save adds the sRGB output intent and the PDF/A identification of the XMP metadata
var pdf = document.ToByteArray();

The save checks what the library can check (no encryption, embedded fonts, a structure tree for the level A, the version, the actions, the annotations, the device color spaces), adds the sRGB output intent, and writes the XMP metadata from the document information with the identification of the level. Set at least a title.

The XMP metadata keeps the properties of the existing packet that the information does not set, such as the custom schema of an electronic invoice and its PDF/A extension schema: see Metadata. A save that changes nothing in the metadata leaves its stream unchanged, so an incremental save does not rewrite it.

What the save checks

Besides the rules of each level in the table below, the save refuses a document that:

• has actions that the level does not allow: Launch, Sound, Movie, ResetForm, ImportData, Hide, Rendition, Trans, SetOCGState and GoTo3DView; JavaScript before PDF/A-4; the named actions other than NextPage, PrevPage, FirstPage and LastPage. The actions of the document, of the pages, of the annotations, of the outline and of the form are checked.
• has additional actions (/AA) on the document, the pages or the fields (PDF/A-4: on the annotations, only those of the cursor and the focus), or an action on a widget.
• has an annotation without appearance (from PDF/A-2; the links, the popups and the annotations without size need none), or with an appearance other than the normal one.
• asks the viewer to generate the appearances of the form (NeedAppearances).
• uses DeviceRGB or DeviceCMYK without an output intent of the same color space or a DefaultRGB or DefaultCMYK color space. DeviceGray goes with any output intent. With the sRGB output intent, use an ICC-based color space for CMYK colors.
• uses a font without embedding it, including a font of the form that a viewer renders a field without appearance with.

The save does not generate the missing appearances: it reports them.

Output intents

A document without output intent gets the sRGB one. A document that already has output intents, such as the one of PDF/X with a CMYK profile, keeps them: the PDF/A output intent shares their profile, as PDF/A requires one profile for every output intent. The profile is then the color space of the output intent: DeviceCMYK goes with a CMYK profile, DeviceRGB does not.

Choose the level

The part (1 to 4) is the version of PDF/A; the letter the level of conformance:

• b (basic): the document shows the same, but its text may not be extractable as Unicode.
• u (Unicode): b, and every text can be extracted as Unicode.
• a (accessible): u, and the document is tagged: see Tagged PDF and PDF/UA.

PDF/A-2 suits most documents: it allows transparency and compression, which PDF/A-1 does not. Choose PDF/A-1 only when an archive requires it, PDF/A-3 or PDF/A-4f to attach files of other formats, and PDF/A-4 for PDF 2.0.

Attach structured data

Electronic invoices often carry their data as an XML file in a PDF/A-3 document: people read the pages, and programs the XML. Give the file its MIME type and its relationship to the document:

var font = PdfFont.Load(fontData);
var document = new PdfDocument { PdfAConformance = PdfAConformance.PdfA3B };
document.Information.Title = "Invoice 2026-001";
document.Pages.Add(PdfSize.A4).CreateContentBuilder().DrawText("Invoice 2026-001", font, 18, 60, 760);

// PDF/A-3 accepts embedded files of any format, with a MIME type and their relationship to the document
document.EmbeddedFiles.Add("invoice.xml", invoiceXml, new PdfEmbeddedFileOptions
{
    Description = "The invoice as structured data",
    MimeType = "application/xml",
    Relationship = PdfAssociatedFileRelationship.Alternative,
    ModificationDate = new DateTimeOffset(2026, 10, 29, 8, 0, 0, TimeSpan.Zero),
});

var pdf = document.ToByteArray();

Alternative says that the file is another representation of the content; Data (the data the content was made from), Source and Supplement are the other relationships. A standard of electronic invoices names the file and the relationship it requires.