← toolkit.bot

WCAG 1.4.10 Reflow: Why PDFs Fail and How EPUB3 Fixes It

WCAG Success Criterion 1.4.10 Reflow is a Level AA requirement that catches most PDFs by surprise. It requires that content be readable at a viewport equivalent to 320 CSS pixels wide — roughly 400% zoom on a 1280px desktop monitor — without horizontal scrolling or loss of information.

For web pages, satisfying 1.4.10 is a matter of responsive CSS. For PDFs, there is no fix. A PDF is a fixed-layout format: pages have physical dimensions baked into the file. At 400% zoom, a standard A4 page is approximately 3,300 pixels wide — over ten times the required viewport — and every user must scroll horizontally to read it.

This article explains why 1.4.10 Reflow is structurally impossible for PDFs, why it matters for EAA and ADA compliance, and why EPUB3 is the correct format for documents that must pass this criterion.

What WCAG 1.4.10 Reflow Requires

WCAG 2.1 and 2.2 Success Criterion 1.4.10 states:

“Content can be presented without loss of information or functionality, and without requiring scrolling in two dimensions for: Vertical-scrolling content at a width equivalent to 320 CSS pixels; Horizontal-scrolling content at a height equivalent to 256 CSS pixels. Except for parts of the content which require two-dimensional layout for usage or meaning.”

In practice, this means a user who zooms a web page or document to 400% on a standard 1280px-wide display must be able to read it in a single scrollable column without needing to scroll left and right. This requirement exists specifically for low-vision users who cannot read text at standard sizes and must zoom into content to read it.

Why PDFs Structurally Cannot Satisfy WCAG 1.4.10

The PDF format was designed for print. Every page in a PDF has fixed physical dimensions — typically A4 (210 × 297mm) or US Letter (8.5 × 11 inches). Those dimensions are embedded in the file specification and cannot be changed by a viewer without distorting the document.

When a user zooms a PDF viewer to 400%, the page becomes proportionally larger. On a 1280px display:

This is not a rendering bug or a tool limitation. It is the fundamental property of a fixed-layout format. No PDF editor — not Adobe Acrobat Pro, not axesPDF, not CommonLook — can make a PDF reflow at 400% zoom in the way WCAG 1.4.10 requires.

What about Acrobat's Reflow View?

Adobe Acrobat Reader has a “Reflow” feature (View → Zoom → Reflow) that attempts to extract text from PDF tags and reformat it as a single flowing column. This is sometimes offered as a solution to 1.4.10.

It is not. Acrobat Reflow has several critical limitations:

For WCAG 1.4.10 conformance purposes, Acrobat Reflow is not a valid solution.

Which Laws and Standards Require WCAG 1.4.10

WCAG 1.4.10 is a Level AA requirement, which means it is required by any law or regulation that mandates WCAG 2.x Level AA conformance:

How EPUB3 Satisfies WCAG 1.4.10 by Design

EPUB3 is built on HTML5 and CSS3 — the same technologies used for accessible web content. Text in an EPUB3 document is reflowable by default. There is no fixed page size. When a reading system increases text size or zooms in, the content reflows to fill the available viewport in a single column.

WCAG 1.4.10 conformance is a structural property of EPUB3, not an optional feature that must be added. A correctly produced EPUB3 document:

The only exception in EPUB3 is Fixed Layout EPUB (FXL) — a subset of EPUB3 designed to preserve page-accurate layout, typically used for picture books and illustrated content. FXL EPUB has the same 1.4.10 problem as PDF. For text-dominant documents, standard reflowable EPUB3 is always the right format.

What This Means for EAA and ADA Compliance

If your organization distributes PDFs as part of a web service, digital publication platform, or online portal, and those PDFs fall within EAA or ADA Title II scope:

Converting PDFs to Reflow-Compliant EPUB3

toolkit.bot/pdf2epub converts PDF documents to EPUB3 in approximately 30 seconds. The output is reflowable EPUB3 with EPUB Accessibility 1.1 metadata — which means it satisfies WCAG 1.4.10 and the other WCAG criteria the original PDF was unlikely to meet.

After conversion, validate with ACE by DAISY (free, npm install -g @daisy/ace) to generate a formal accessibility report. The ACE report explicitly checks WCAG 1.4.10 in the EPUB context and provides a machine-readable JSON summary suitable for compliance documentation.

Convert your PDF to reflowable EPUB3 — satisfies WCAG 1.4.10 by design. Free, no account required.

Convert a PDF →   EAA compliance guide →

Related guides