The one-line distinction
- NIMAS is a source format. Nobody reads it directly; it is converted into student-ready formats.
- PDF/UA is an end-user format. The reader opens the file itself with a screen reader, magnifier or braille display.
- EPUB is an end-user format built for reflow. Text adapts to the reader's screen, font size and preferences.
When NIMAS is the answer
When a US K–12 contract requires it. NIMAS is not a general accessibility measure you would adopt voluntarily for a trade title — it is a specific obligation attached to instructional materials, and its whole purpose is to feed the conversion pipeline that produces braille and other specialist formats.
Producing NIMAS does nothing for the accessibility of the file your customers download. That is a separate question.
When PDF/UA is the answer
When the PDF is what people actually use — because your product is distributed as a PDF, or because a buyer's procurement process asks for conformance evidence on the file you ship.
PDF/UA (ISO 14289) is about the file having a real tag tree: headings, reading order, table headers, alternatives, language, bookmarks. A visually correct PDF with no tags is unusable with a screen reader, however clean it looks.
Its limitation is reflow. A PDF has a fixed page geometry, and while modern readers can reflow tagged content, it is not the format's strength. A low-vision reader at high magnification is generally better served by EPUB.
When EPUB is the answer
When the reading experience matters at the reader's own settings — long-form text, adjustable type size, reading on a phone, or distribution through retail and library channels that expect EPUB.
EPUB Accessibility 1.1 also lets you declare a title's accessibility in its metadata, which is what makes accessible titles discoverable in catalogues rather than merely present in them. For a lot of publishers this is now the practical differentiator.
Do you need more than one?
Frequently, yes — and that is less painful than it sounds. All three can generally be produced from the same source material, and the expensive, judgement-heavy work of writing image descriptions is done once and reused across every output.
The sequence that usually costs least: establish the structure and the descriptions once, then generate the formats each channel requires from it.
A note on ACRs
Whichever format you ship, a buyer may ask for an Accessibility Conformance Report against it. Reports are written per product and per format, so a title distributed as both PDF and EPUB may need conformance evidence for each.
Working on something this touches? Send us the details and we will tell you plainly what it takes.