Understanding EPUB → PDF
Reflowable to fixed — the conversion that loses what made ebooks ebooks.
Why ebooks and PDFs are fundamentally different shapes, what survives the conversion, and the page-size decisions that matter.
Reflowable vs paginated.
EPUB is reflowable: the same content adapts to the screen, font size, and reading orientation. PDF is paginated: every page is fixed at print-design time, regardless of the device. Converting EPUB to PDF is therefore destructive — you pick a page size, a font, a margin, and bake all of those into a non-resizable document. The reader can no longer change them.
EPUB internals.
An EPUB is a ZIP file containing HTML, CSS, images, and an OPF manifest listing everything plus reading order. The reader (Adobe Digital Editions, Apple Books, Kobo) renders the HTML with embedded CSS, scaled to the device. The same file looks different on a phone, a tablet, and an e-reader — by design. Convert to PDF and you lose that.
How the conversion works.
Most converters use a headless browser (Chromium via Puppeteer, WeasyPrint, Prince). Read the OPF, concatenate the HTML files in spine order, apply the CSS, render to paginated output, save as PDF. The trickier work is heading-to-bookmark conversion (the EPUB's nav.xhtml becomes the PDF's table of contents), font subsetting (only ship the glyphs actually used), and image scaling for the chosen page size.
Page size matters.
Pick US Letter or A4 (default), the PDF prints well but reads awkwardly on a phone. Pick something narrow like 6 × 9 inches (trade paperback), it reads better on mobile but wastes paper. Most converters offer a "phone reading" preset and a "print paperback" preset; the page-size decision drives margin and font-size decisions downstream. A choice you can't undo without re-running.
A worked conversion.
A 300-page novel as EPUB (~800 KB with images). Converted to A4 PDF at 11pt serif with 0.75-inch margins: ~250 pages, 2.5 MB. Reflow-aware features lost (font size choice, line-height adjustment). Print-friendly. The same EPUB to 6×9 trade paperback at 10pt: 400 pages, 3 MB, fits a 6-inch e-reader screen well. Same source, two destinations.
300-page novel
EPUB → A4 PDF
Pick page size, render with headless browser.
800KB EPUB → 250-page 2.5MB PDF
= Loses reflow, gains pagination
When not to convert.
If the destination is a device that supports EPUB natively (Kindle Paperwhite via Send-to-Kindle now accepts EPUB, every iOS device via Books app, every Android via dozens of readers), keep the EPUB. PDF makes sense for printing, for archival, for devices that only render PDF, or for documents that need fixed layouts (technical diagrams, legal contracts). Convert for the right reason; the reverse direction rarely produces a good EPUB.