MOBI is the old Kindle format, and a very poor carrier of formatting. PDF is a frozen page layout with no heading structure a machine can rely on. calibre sits between the two, and this page says exactly what happens to a MOBI file on the way to becoming a PDF one.
What runs when a MOBI file becomes PDF
calibre the ebook-convert binary, on a machine we rent and watch. Your MOBI file is uploaded once, calibre runs once, the PDF comes back, and neither file is kept. MOBI to PDF 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 MOBI into the PDF
The reading order and the chapter split of the MOBI file. calibre rebuilds it as a book on the way to PDF, rather than as a pile of pages.
Images from the MOBI file, placed inline in the PDF where the text refers to them.
What MOBI to PDF costs you
Your page breaks, because a MOBI file never had any. calibre reflows the text first and computes PDF pages from the result, so where a page ends is the engine decision and not the author one.
If a model is going to read the PDF
Worth knowing before the PDF goes anywhere near an assistant: this is the wrong direction for that. A PDF is a set of positioned glyphs, and a model has to reconstruct the reading order before it can begin. If a machine rather than a person is going to read this MOBI file, convert it to Markdown instead and skip the reconstruction entirely.
MOBI to PDF, measured rather than promised
Running MOBI to PDF against the real engine with a real file, calibre wrote 62,207 bytes of PDF in 1.9 seconds, machine otherwise idle. That is one MOBI file on one day and not an average, which is why the number is given together with the file that produced it.
The MOBI to PDF 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 PDF file has anything inside it.
The same MOBI file, sent somewhere else
Other outputs the probe measured out of a MOBI file, so the cost of choosing PDF can be read against something. Sizes do not compare across engines, because each family was probed with its own MOBI witness file.
MOBI to DOCX: 24,070 bytes in 0.6 seconds, verified by a format signature.
MOBI to EPUB: 22,644 bytes in 0.6 seconds, verified by a format signature.
MOBI to TXT: 219 bytes in 0.5 seconds, verified by reading the text back.
MOBI to RTF: 2,489 bytes in 0.6 seconds, verified by a format signature.
MOBI to AZW3: 61,343 bytes in 0.6 seconds, verified by a format signature.
Other ways into PDF, and what they measured
Among the published routes into PDF, MOBI is the eighth largest output of the 26 measured. The witness files differ, so this ranks the probe run and not your document.
AZW3 to PDF: 62,065 bytes in 1.8 seconds.
HTML to PDF: 25,178 bytes in 1.9 seconds.
TXT to PDF: 12,976 bytes in 1.8 seconds.
JPG to PDF: 4,232 bytes in under a tenth of a second.
PNG to PDF: 2,536 bytes in under a tenth of a second.
What we will not pretend about MOBI to PDF
A MOBI file over 50 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A MOBI to PDF 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 MOBI 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 PDF timing below is a measurement of one small file and not a guarantee about your thousand page one.
One MOBI file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
MOBI to PDF: what people ask
What actually converts my MOBI file to PDF?
calibre does it, the ebook-convert binary, on a machine we rent and watch. Not a browser trick and not somebody else service: the MOBI file is uploaded once, calibre runs once, the PDF comes back, and neither file is kept afterwards.
How long does MOBI to PDF take?
On the file the probe used, calibre took 1.9 seconds and wrote 62,207 bytes of PDF. That is one real measurement on one real MOBI file, not an average and not a promise about yours: a larger MOBI takes longer, and past 120 seconds the run is stopped.
What do I lose going from MOBI to PDF?
The one to know about first: Your page breaks, because a MOBI file never had any. calibre reflows the text first and computes PDF pages from the result, so where a page ends is the engine decision and not the author one.
Does the table of contents survive from MOBI to PDF?
It does, as long as your MOBI file has real headings rather than text that merely looks like headings. calibre builds the PDF contents from the heading structure it finds, and hand-formatted big bold lines are not a structure.
Is MOBI to PDF free?
There is a free allowance every month, and one MOBI to PDF 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 PDF.