PDF is a frozen page layout with no heading structure a machine can rely on. EPUB is a reflowable book split into chapters. calibre sits between the two, and this page says exactly what happens to a PDF file on the way to becoming an EPUB one.
What runs when a PDF file becomes EPUB
calibre the ebook-convert binary, on a machine we rent and watch. Your PDF file is uploaded once, calibre runs once, the EPUB comes back, and neither file is kept. PDF to EPUB is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the PDF into the EPUB
The reading order and the chapter split of the PDF file. calibre rebuilds it as a book on the way to EPUB, rather than as a pile of pages.
Cover art and book metadata, which calibre carries from the PDF container into the EPUB one.
What PDF to EPUB costs you
Fixed layout. calibre reflows the PDF text by design, because an EPUB is meant to be read at any font size on any screen width.
Anything the PDF did not spell out. calibre reads the text layer of the PDF file; a scanned page has no text layer, so a scanned PDF comes back as an empty EPUB, and no engine on this machine can warn you beforehand.
If a model is going to read the EPUB
Structure is what survives from the PDF file, and structure is what a model needs. PDF in, EPUB out, with the heading tree intact instead of flattened into one long paragraph.
PDF to EPUB, measured rather than promised
Running PDF to EPUB against the real engine with a real file, calibre wrote 26,909 bytes of EPUB in 0.9 seconds, machine otherwise idle. That is one PDF file on one day and not an average, which is why the number is given together with the file that produced it.
The PDF to EPUB verdict was reached by a format signature, meaning at least four bytes unique to the format were found at the right offset. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the EPUB file has anything inside it.
The same PDF file, sent somewhere else
Other outputs the probe measured out of a PDF file, so the cost of choosing EPUB can be read against something. Sizes do not compare across engines, because each family was probed with its own PDF witness file.
PDF to DOCX: 23,905 bytes in 0.9 seconds, verified by a format signature.
PDF to JPG: 5,782 bytes in 0.1 seconds, verified by a format signature.
PDF to PNG: 1,294 bytes in 0.1 seconds, verified by a format signature.
PDF to WEBP: 906 bytes in 0.1 seconds, verified by a format signature.
PDF to TXT: 219 bytes in 0.8 seconds, verified by reading the text back.
PDF to GIF: 9,463 bytes in 0.1 seconds, verified by a format signature.
Other ways into EPUB, and what they measured
Among the published routes into EPUB, PDF is the first largest output of the 11 measured. The witness files differ, so this ranks the probe run and not your document.
MD to EPUB: 5,034 bytes in 0.1 seconds.
DOCX to EPUB: 5,040 bytes in 0.1 seconds.
CSV to EPUB: 4,913 bytes in 0.1 seconds.
HTML to EPUB: 5,027 bytes in 0.1 seconds.
JSON to EPUB: 5,035 bytes in 0.1 seconds.
What we will not pretend about PDF to EPUB
A PDF file over 50 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A PDF to EPUB run that passes 120 seconds is killed, and the calibre process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
The PDF file used to prove this pair was short. calibre is allowed 120 seconds and a long book can genuinely need all of them, so the EPUB timing below is a measurement of one small file and not a guarantee about your thousand page one.
One PDF file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
PDF to EPUB: what people ask
What actually converts my PDF file to EPUB?
calibre does it, the ebook-convert binary, on a machine we rent and watch. Not a browser trick and not somebody else service: the PDF file is uploaded once, calibre runs once, the EPUB comes back, and neither file is kept afterwards.
How long does PDF to EPUB take?
On the file the probe used, calibre took 0.9 seconds and wrote 26,909 bytes of EPUB. That is one real measurement on one real PDF file, not an average and not a promise about yours: a larger PDF takes longer, and past 120 seconds the run is stopped.
What do I lose going from PDF to EPUB?
The one to know about first: Fixed layout. calibre reflows the PDF text by design, because an EPUB is meant to be read at any font size on any screen width.
Does the table of contents survive from PDF to EPUB?
It does, as long as your PDF file has real headings rather than text that merely looks like headings. calibre builds the EPUB contents from the heading structure it finds, and hand-formatted big bold lines are not a structure.
Is PDF to EPUB free?
There is a free allowance every month, and one PDF to EPUB conversion costs half a credit against it. When the allowance runs out the tool says so and stops, rather than quietly handing you a worse EPUB.