PDF conformance standards, and which ones this API produces

The oldest and strictest archiving profile: a PDF 1.4 file with everything it needs to render sealed inside it, and nothing in it that a future reader might not understand.

Everything PDF/A-1b requires, plus a complete logical structure tree, a declared document language and a Unicode mapping for every glyph, so the file is readable by a machine and not only by an eye.

The second part of the archiving standard, rebased on PDF 1.7, which quietly permits most of the things PDF/A-1 banned and is therefore the realistic target for a document produced from a stylesheet.

Level b plus one extra rule: every character in the document must map to a Unicode value, so the text can be searched, indexed and copied rather than merely looked at.

Part 2 with one rule removed: an embedded file no longer has to be a PDF/A file itself, which is what makes a hybrid invoice carrying both a printable page and a machine-readable XML payload possible.

The 2020 rebase onto PDF 2.0, which drops the a, b and u conformance letters entirely, makes Unicode mapping mandatory for everyone, and splits the optional extras into two named variants instead.

PDF/A on its own is not a specification: it is four parts crossed with up to three conformance levels, and the combination decides whether your job is a converter setting or a month of manual tagging.

A dictionary in the document catalogue holding an embedded colour profile, which is what turns an otherwise meaningless triple of numbers into a colour a future reader can reproduce.

Every conformance standard requires the actual font program to be inside the file, because a font named but not embedded is a promise that some other machine will have it, and archiving exists precisely because that promise fails.

Conformance is a property of the bytes, not of the metadata that claims it, and the only way to know which one you have is to run the file through a validator that reports clauses.

ISO 14289-1: the standard that says a PDF is usable by assistive technology, which in practice means it carries a complete and correct structure tree.

A tagged PDF carries a second, invisible copy of itself: a tree of headings, paragraphs, lists and cells that says what the content is, rather than where the ink goes.

The sequence a screen reader speaks the document in, which comes from the order of elements in the structure tree and not from where anything sits on the page.

A text description attached to a figure structure element, which is what a screen reader speaks in place of an image it cannot see.

A language tag in the catalogue that tells a screen reader which voice and pronunciation rules to use for the document, and per-element tags for passages that differ.

The tree of typed elements, Document at the root and paragraphs, lists, tables and figures below it, that describes what the document is made of.

Marking which cells are headers and which data cells they govern, so a screen reader can announce "Quantity, 4" instead of reading a grid of bare numbers.

The /H1 to /H6 structure elements that give a document an outline, which is how a screen reader user skims a fifty page report without reading it.

The distinction between content that belongs in the reading order and page furniture that does not, marked in the content stream so a screen reader can skip the furniture.

The blind exchange standard for offset printing: everything is CMYK or a named spot colour, nothing is transparent, and the file carries enough information that the printer never has to ask you a question.

The same handover discipline as X-1a with one rule relaxed: device independent colour is allowed, so ICC tagged RGB and Lab content can stay in the file and be converted by the printer's own colour management.

The current print exchange standard, based on PDF 1.6, which keeps transparency and layers live in the file and lets the printer's own engine flatten them at the last possible moment.

Print uses inks, the web uses light, and there is no CSS syntax that expresses either a CMYK ink mixture or a named separation, so the gap has to be closed somewhere other than the stylesheet.

A printed page has at least three rectangles: the sheet, the finished size after cutting, and the slightly larger area artwork must fill so that a cut a millimetre off does not leave a white sliver.

Two different ways of putting one thing on top of another: overprint is an ink instruction that a screen mostly ignores, transparency is a rendering model that older print standards ban outright, and confusing them produces the classic disappearing white text.

An XML packet in the document catalogue carrying the file's own account of what it is: title, author, dates, and any conformance it claims.

The older metadata mechanism: a dictionary in the trailer holding /Title, /Author, /Subject, /Keywords and the creation and modification dates.

The version in the file header and optionally in the catalogue, which says which features a reader must understand to open the file correctly.

ISO 32000-2, the first version of PDF published by ISO without an Adobe specification behind it, and the base for PDF/A-4.

A file laid out so a reader can display page one before the rest has arrived, which is what "fast web view" means in a viewer's properties panel.

A form field holding a cryptographic signature over the file, which proves who signed it and that nothing has changed since.

The first signature on a document, which certifies the whole thing and declares what, if anything, anyone is allowed to change afterwards.

The four European profiles for PDF signatures, each adding what the one below it needs to still be verifiable years later.

Why a password-protected file can never be PDF/A or PDF/UA, and what the password option here actually gives you.

The mechanism behind hybrid invoices: a machine-readable XML payload carried inside an archivable PDF, so one file serves both the person and the system.

Interactive form fields inside a PDF, and why an HTML input rendered to PDF is a picture of a field rather than a field.