EPUB3 vs PDF: Accessibility Comparison
PDF is the universal document format. EPUB3 is the accessible document format. This comparison explains why they serve different purposes — and why documents that need to be both widely distributed and accessible need both.
The Core Difference
PDF (Portable Document Format) was designed for print fidelity. Content is fixed to page dimensions — a PDF looks identical whether you print it, display it at 100% zoom, or open it on any device. This makes PDF ideal for documents that need to look exactly right: regulatory submissions, signed contracts, print-ready publications.
EPUB3 was designed for reading. Built on HTML5, CSS, and SVG, EPUB3 content reflows to the available viewport width and adapts to the reader’s font size and zoom settings. An EPUB3 document looks different on a phone, a tablet, and a wide-screen monitor — because it adapts to each.
That difference is also an accessibility difference.
Accessibility Comparison by Criterion
WCAG 1.4.10 Reflow (the critical criterion)
| EPUB3 | ||
|---|---|---|
| Can content reflow at 400% zoom? | ✗ No — fixed dimensions | ✓ Yes — HTML5 reflow |
| Multi-column layouts at high zoom | ✗ Requires horizontal scrolling | ✓ Single-column reflow |
| Complex tables at high zoom | ✗ Fixed-width table columns | ✓ Responsive table rendering |
| Can be fixed by tagging? | ✗ No — format property | N/A — satisfied structurally |
Why PDF cannot satisfy WCAG 1.4.10: When a PDF page is created, it has fixed dimensions (typically 595pt × 842pt for A4). At 400% zoom, each page dimension multiplies by 4. Content doesn’t reflow to the screen; the page becomes very large and horizontal scrolling is required. This is true regardless of tagging quality. It is a property of the format, not a remediable defect.
Why EPUB3 satisfies WCAG 1.4.10 structurally: EPUB3 content is HTML5. At any zoom level, text wraps to the screen. No horizontal scrolling required.
WCAG 1.3.1 Information and Relationships
| EPUB3 | ||
|---|---|---|
| Heading structure | ✓ Via PDF/UA tagging | ✓ Native HTML heading elements |
| Reading order | ✓ Via PDF/UA tagging | ✓ Native HTML document order |
| Table markup | ✓ Via PDF/UA tagging | ✓ Native HTML table elements |
| Requirement to achieve | Requires PDF/UA remediation | Inherent in well-authored EPUB3 |
WCAG 1.1.1 Non-text Content (alt text)
| EPUB3 | ||
|---|---|---|
| Image alt text support | ✓ Via PDF/UA ActualText/Alt | ✓ Native HTML alt attribute |
| Decorative image marking | ✓ Via artifact tagging | ✓ alt="" or role="presentation" |
| Complex image descriptions | ✓ Via /ActualText | ✓ aria-describedby or <figcaption> |
WCAG 1.4.4 Resize Text
| EPUB3 | ||
|---|---|---|
| Text resize without horizontal scroll | ✗ Fails at high zoom | ✓ Text reflows |
| Font size user control | ✗ Zoom only (maintains layout) | ✓ Reading system font size control |
| Reading system font override | ✗ Fixed fonts in PDF | ✓ User-adjustable fonts in EPUB3 |
Screen Reader Compatibility
| Reading system | PDF support | EPUB3 support |
|---|---|---|
| NVDA + Firefox | PDF.js rendering (variable) | Thorium Reader — excellent |
| JAWS | Adobe Reader or Foxit — good | Thorium Reader — excellent |
| VoiceOver (macOS) | Preview or Safari PDF — good | Apple Books — excellent |
| VoiceOver (iOS) | Files app — variable | Apple Books — excellent |
| TalkBack (Android) | Variable | ReadEra, Moon+ Reader — good |
When to Use Each Format
Keep PDF for:
- Regulatory submissions (EMA, FCA, SEC filings)
- Signed contracts with digital signatures
- Print-ready documents (marketing brochures, physical packaging inserts)
- Documents that must look identical across all devices
- Legal instruments where format integrity is required
Use EPUB3 for:
- All documents distributed digitally for reading
- Documents that must satisfy WCAG 1.4.10 (required under EAA, EN 301 549, UK Accessibility Regulations)
- Books, long-form publications, documentation
- Patient-facing documents (PILs, hospital guides)
- Consumer-facing regulatory documents (KIDs, IPIDs, T&Cs)
Use both (dual-format distribution):
- Any document that currently exists as PDF and must now be accessible
- Publish the PDF for print/archival purposes; publish the EPUB3 as the accessible reading version
What PDF/UA Achieves (and Doesn’t)
PDF/UA (ISO 14289) is the PDF accessibility standard. A PDF/UA-compliant document satisfies most WCAG 2.1 criteria — but not WCAG 1.4.10 Reflow. The PAC 2024 accessibility checker explicitly does not test WCAG 1.4.10 because PDF cannot satisfy it.
The EAA Implication
The European Accessibility Act (enforcement active June 28, 2026) requires WCAG 2.1 AA for financial services documents, e-book publishers, telecom customer documents, transport booking, and government documents via EN 301 549. WCAG 2.1 AA includes WCAG 1.4.10 Reflow. Fixed-layout PDF cannot satisfy it. EPUB3 can.
Organisations in these sectors need to provide accessible EPUB3 versions of documents they currently distribute as PDF only.
Validator Comparison
| Tool | Tests PDF/UA | Tests WCAG 1.4.10 | Tests EPUB Accessibility 1.1 |
|---|---|---|---|
| PAC 2024 | ✓ | ✗ | ✗ |
| Acrobat Accessibility Check | Partial | ✗ | ✗ |
| ACE by DAISY | ✗ | ✓ (for EPUB3) | ✓ |
| Axe (browser extension) | ✗ | ✓ (for web/EPUB3) | Partial |
Converting PDF to Accessible EPUB3
toolkit.bot converts PDF to accessible EPUB3 in 30 seconds. Free tier at /pdf2epub. API at /pricing.
ACE by DAISY validation confirms the output:
npm install -g @daisy/ace
ace --outdir ./report ./document.epubThe report confirms WCAG 1.4.10 Reflow pass — the criterion PDF cannot satisfy.
Frequently asked questions
Can PDF satisfy WCAG 1.4.10 Reflow?
No. WCAG 1.4.10 requires content to be readable at 320px viewport width without horizontal scrolling. PDF is a fixed-layout format — at 400% zoom the page expands rather than reflowing. This is a format property, not a tagging problem. Even a fully PDF/UA-compliant document cannot satisfy WCAG 1.4.10. EPUB3, built on HTML5, satisfies 1.4.10 structurally.
Does PDF/UA satisfy EAA requirements?
PDF/UA satisfies most WCAG criteria but explicitly does not test WCAG 1.4.10 Reflow — the PAC 2024 checker documents this. The European Accessibility Act requires WCAG 2.1 AA including 1.4.10. PDF/UA alone is therefore not sufficient for EAA compliance. EPUB3 is the accessible format that satisfies WCAG 2.1 AA including Reflow.
Which screen reader works best with EPUB3?
Thorium Reader (free, Windows/Mac/Linux) is the reference EPUB3 reading system for accessibility and works excellently with NVDA and JAWS. Apple Books works well with VoiceOver on macOS and iOS. EPUB3 screen reader support is generally more predictable than PDF because EPUB3 content is HTML5 — the same technology screen readers navigate daily on the web.
What is the difference between PAC 2024 and ACE by DAISY?
PAC 2024 tests PDF against PDF/UA (ISO 14289). ACE by DAISY tests EPUB3 against EPUB Accessibility 1.1 and WCAG 2.x. For EAA compliance documentation, ACE by DAISY is the reference validator — it tests WCAG 1.4.10 Reflow and produces reports suitable for regulatory audit evidence. PAC 2024 does not test WCAG 1.4.10.
When should I keep PDF and when should I use EPUB3?
Keep PDF for regulatory submissions requiring exact layout (SEC filings, signed contracts, print-ready documents). Use EPUB3 for any document distributed digitally for reading — documents that must satisfy WCAG 1.4.10 under EAA, EN 301 549, or UK Accessibility Regulations. For documents currently distributed as PDF that must now be accessible, dual-format distribution (PDF for archival + EPUB3 as the accessible reading version) is the practical path.