PDF is a frozen page layout with no heading structure a machine can rely on. MOBI is the old Kindle format, and a very poor carrier of formatting. calibre sits between the two, and this page says exactly what happens to a PDF file on the way to becoming a MOBI one.
What runs when a PDF file becomes MOBI
calibre the ebook-convert binary, on a machine we rent and watch. Your PDF file is uploaded once, calibre runs once, the MOBI comes back, and neither file is kept. PDF to MOBI 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 MOBI
The reading order and the chapter split of the PDF file. calibre rebuilds it as a book on the way to MOBI, rather than as a pile of pages.
Cover art and book metadata, which calibre carries from the PDF container into the MOBI one.
What PDF to MOBI costs you
Fixed layout. calibre reflows the PDF text by design, because a MOBI 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 MOBI, and no engine on this machine can warn you beforehand.
Heading tags, and this is measured rather than assumed: we put an EPUB carrying a heading, a table, bold text, a colour and a font through every ebook target and read each result back with calibre. The MOBI does not carry heading tags at all. What a device then DISPLAYS is a separate question, and one no measurement here reaches.
If a model is going to read the MOBI
The words all survive, and the heading STRUCTURE does not: MOBI has no tag for a heading, so calibre writes them as styled paragraphs. A model reading the MOBI gets the text in order and has to infer the sections from it, which is exactly the work PDF was saving it. If a model is the reader, convert to Markdown instead.
PDF to MOBI, measured rather than promised
Running PDF to MOBI against the real engine with a real file, calibre wrote 57,343 bytes of MOBI 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 MOBI 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 MOBI 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 MOBI 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 EPUB: 26,909 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.
Other ways into MOBI, and what they measured
Among the published routes into MOBI, PDF is the fourth largest output of the 9 measured. The witness files differ, so this ranks the probe run and not your document.
DOCX to MOBI: 59,876 bytes in 0.8 seconds.
AZW3 to MOBI: 59,812 bytes in 0.8 seconds.
HTML to MOBI: 9,294 bytes in 0.6 seconds.
MD to MOBI: 9,300 bytes in 0.7 seconds.
ODT to MOBI: 9,422 bytes in 0.9 seconds.
What we will not pretend about PDF to MOBI
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 MOBI 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.
Your headings stop being headings. MOBI has no tag for one, so calibre writes them as styled paragraphs. The table of contents is built before that happens and survives; what you lose is the structure INSIDE the text, which matters the moment something reads the file rather than displays it.
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 MOBI 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 MOBI: what people ask
What actually converts my PDF file to MOBI?
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 MOBI comes back, and neither file is kept afterwards.
How long does PDF to MOBI take?
On the file the probe used, calibre took 0.9 seconds and wrote 57,343 bytes of MOBI. 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 MOBI?
The one to know about first: Fixed layout. calibre reflows the PDF text by design, because a MOBI is meant to be read at any font size on any screen width.
Does the table of contents survive from PDF to MOBI?
It does, as long as your PDF file has real headings rather than text that merely looks like headings. calibre builds the MOBI contents from the heading structure it finds, and hand-formatted big bold lines are not a structure.
Is PDF to MOBI free?
There is a free allowance every month, and one PDF to MOBI 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 MOBI.