The EPUB will not open.

An EPUB isn't a document you double-click like a PDF — it needs an e-reader app. If it won't open, you're probably handing it to the wrong program.

Open it in one of these:

• Apple Books (Mac/iPhone/iPad) — drag it in.
• Calibre (free, Windows/Mac/Linux).
• Thorium Reader (free).
• Kindle Previewer (free from Amazon) — to see how it'll look on Kindle.

See How do I test the EPUB before publishing? If it opens but looks different from the PDF, that's expected — an EPUB reflows. See Why does my EPUB look different on different devices?