PDF/A-1: the first archiving standard, and what it settled

In plain words

Written for anybody, with nothing taken for granted. It says what the rule is for and what it changes on the document you end up with.

In 2005 a PDF could name a typeface it did not carry, describe a colour as whatever this particular screen called red, and point at a picture held somewhere else. All of it worked, and all of it stopped working the day the typeface, the screen or the address was gone.

So a set of rules was written that shut every one of those doors at once. It is short, it is strict, and it is the reason a document from 2006 written this way still opens today in Adobe Acrobat Reader (Windows, Mac, Android, iPhone), in Preview on the Mac, in Chrome, Firefox and Edge, in Okular and Evince on Linux and in SumatraPDF on Windows.

It came in two levels. The plainer one promises the page: it will look the same for as long as the file exists. The fuller one promises the page and its meaning: the document says which text is a heading and which is a paragraph, so software reading it aloud says it in the right order.

For a developer

The same thing without the detour: the objects, the constraints, and where the line falls between what is produced and what is declared.

The part rests on PDF 1.4, the version of the format Adobe published before the format became an ISO standard of its own. That is what dates it: everything the format gained afterwards is outside this part by construction, which is why the later editions exist.

Two levels, written as a letter after the number. Level B asks for the page to be reproducible; level A asks for that and for a structure hierarchy, a mapping from every drawn character back to Unicode, and the language of the text to be stated.

The claim is stated in the document's own description block, in two entries: the part and the level. A file whose description says one thing and whose contents say another is not a matter of opinion — a checker reads both.

Clause by clause

The published text behind each requirement, what the engine measures the document against, and what it leaves to a validator.

This part is not among the claims the engine writes today, and it is planned for a future version. Most of what it asks for is already produced for the parts that are written — the lettering, the colour statement, the description block — and what stands between the two is the set of constructs this part alone closes off, listed below.

A document that has to be claimed as this part today is best written as one of the later ones and checked against the requirement: the later parts keep every promise this one makes, and an archive that names the family rather than the edition accepts them.

The levels of this part

What this part asks for

• Every typeface the document draws with travels inside the file.
• A colour that depends on the device is accompanied by a statement saying which device, so it can be reproduced rather than guessed at.
• The document carries a description block naming the part and the level it claims.
• Every annotation carries the drawing of how it is to appear.

What this part rules out

A rule that closes a door is what makes the promise keepable: each of these is a way a document could stop being readable later.

• Encryption of any kind: an archived file opens without a password.
• Anything the document does not carry itself, whether a typeface, a picture or a page fetched from an address.
• Programs inside the document, and anything that launches a program.
• Sound, film and anything else that is not ink on a page.
• See-through paint and layers that can be switched off: both arrived in the format after this edition was written.
• A file carried inside the document: the second edition allows that.

Where this part stands here today

The claim itself is planned for a future version. What it asks of the lettering, the colours and the description block is already what comes out of a document claiming one of the parts written today.

What every archival document here carries

This holds whichever part is claimed. Nothing on it has to be asked for a second time.

• Every letter the document draws travels inside it, cut down to the shapes the document actually uses, so the page reads the same on a machine where that typeface was never installed.
• A colour statement is written whether or not one was asked for, so the greys and the reds mean the same thing on the next screen as on this one.
• The block that says what the file is — its title, the moment it was written, the part it claims and the year of that edition — is written into the document, and the claim is stated where the part being claimed expects to find it.
• A report can be asked for before anything is written: it comes back with every finding at once, each naming the published text it answers to and the font, layer, page, link, field or cell it is about.
• A document that claimed a part and breaks one of its rules is refused rather than written wrong, and the refusal names the rule.
• Everything the document leads to is inside the document. Every part of the standard allows an instruction opening a file that sits beside it instead; the team declines that instruction under an archival claim, so the document stays readable on its own the day the folder around it is gone. It is the same line the engine already draws on forms.
• Every archival document published on this site is put through two checkers that had no hand in writing it, veraPDF and Ghostscript, and it is their verdict that is shown — not ours.

The requirements, family by family

The eight families below are the same set of requirements for every part, and each of their pages says what comes out on its own and what only you can supply.

• Which part of the archiving standard to claim The archiving standard is a family, not a single rule. Four of its members can be claimed here, and choosing between them takes two questions.
• Every letter the document draws travels inside it A document that carries its own lettering opens the same way on a machine that has never had that typeface installed. That is the first thing the standard asks for, and the one that changes the file most.
• Colours that mean the same thing on every device Numbers on a page are not colours until something says what device they were chosen for. An archival document carries that statement inside it.
• A page that comes out the same for everyone The promise of an archival document is that two readers, twenty years apart, see the same page. That holds only where nothing on the page is left for the reader to settle.
• The document says what its marks stand for A page of ink says where the marks went and nothing about what they mean. The level that is read aloud, and the accessibility standard, ask the document to say it.
• A document that carries other files inside it Part 3 exists because a document sometimes has to carry another file inside it, and that file has to come back out years later, whole.
• What the file says about itself A document kept for thirty years is found again by what it says about itself, not by the folder it happened to be in.
• Checking the claim, before and after Claiming a standard is one line inside the file. What makes the claim worth anything is that something independent agrees with it.

The published texts behind it

Glossary

standard

A rule argued out in committee, published under a number anybody may buy and read, and identical for every firm claiming it. A claim to follow one can therefore be checked against the text. A way of working that merely spread because it worked is a habit of the trade: useful, widespread, and answerable to no text at all.

veraPDF

A free program that checks whether a file really is what it claims to be, against the published rules. It is the tool archives use, so it is the one used here: a document is not called compliant because we say so, but because veraPDF passed it.

attachment

A whole file tucked inside a PDF and carried along with it, the way a paper clip holds a sheet to a folder. Opening the document is enough to get it back out. An electronic invoice works this way: the page a person reads carries the same amounts as a file accounting software reads.

output intent

The colour profile written into a document to say which press or which screen its colours were chosen for. A printer reads it to know what the numbers in the file are meant to look like, and the rules on documents kept for the long term ask for one.

CMYK

The four inks a printing press lays down: cyan, magenta, yellow and black. A screen mixes light instead, so a document going to a press states its colours in inks, not in screen colours.

font

The drawing of every letter, digit and mark a document writes, held in a file of its own. A PDF carries the fonts it is set in inside itself, which is why it opens looking the same on a machine that has never had them. Leave them out and a reader puts another font in their place, and the layout moves.

structure tree

The list, held inside a tagged PDF, that says what each part of the page is and what it belongs to: this block is a section, this line its heading, this run of cells one row of that table. It is the shape of the document, kept apart from the ink.

layer

A named group of things drawn on a page that a reader can switch on and off, like tracing paper laid over a plan. A drawing can hold its measurements on one and its notes on another, and a layer can be set to show on screen and stay off the printer.

annotation

Something laid over a page rather than drawn into it: a clickable rectangle, a box to type in, a button. It sits at its own place on the sheet and is what makes a PDF answer to a click at all.

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