PDF/A
PDF/A is a PDF that carries everything it needs inside itself, so that opening it decades from now does not depend on anything else still being around.
In more detail
An ordinary PDF is allowed to point outside itself. It can name a font and expect the machine reading it to own a copy. It can state a colour as three numbers and leave the screen to decide what red that is. Decades from now the font is gone and the screen is another one, and the page no longer looks like the page that was signed.
PDF/A closes every one of those doors. The lettering travels inside the file. The colours say which device they were chosen for. The file describes itself, in a metadata packet a machine can read, so a computer that has never met it knows what it is holding. Nothing it needs is somewhere else.
What it does on a real document
Take an invoice. In an ordinary PDF the total may be drawn with a font the file merely names. In a PDF/A the drawing of every letter used is copied into the file — and only the letters that are actually drawn, so the invoice is not fatter for it.
The same file carries a short identity card: its title, when it was made, and the exact rule it claims to follow. veraPDF reads that card first, then goes through the whole file to see whether the claim holds.
Where it comes from, and why
The rule was published as an international standard in 2005, under the number ISO 19005. Courts, archives and public bodies were the ones asking for it: they keep documents for decades, and they cannot reopen the question every time a program changes.
It has grown in parts since. The second part, from 2011, follows a later version of the PDF format. The third, from 2012, is the one that lets a document carry other files attached inside it, which is exactly what an electronic invoice needs. A fourth part came in 2020, for PDF 2.0.
See it on a finished document
An archivable PDF/A-3 form Factur-X: the electronic invoice The page that goes further on this word