PDF is a frozen page layout with no heading structure a machine can rely on. ICO is a Windows icon, capped at 256 pixels a side by the file structure itself. ImageMagick sits between the two, and this page says exactly what happens to a PDF file on the way to becoming an ICO one.
What runs when a PDF file becomes ICO
ImageMagick 6.9.12, on a machine we rent and watch. Your PDF file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept. PDF to ICO 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 ICO
Every pixel value. PDF and ICO are both lossless, so the ICO file adds no compression artefacts that the PDF did not already have.
What PDF to ICO costs you
Scalability. PDF describes shapes, ICO stores a grid of pixels, and the engine rasterises at 150 dots per inch on the way across. That density is a deliberate choice: at the 72 dpi ImageMagick defaults to, an A4 PDF page comes out 595 pixels wide and reads as a failed conversion. Enlarge the ICO afterwards and it blurs, because there is nothing left to redraw from.
Every page of the PDF but the first. A PDF file can hold several images in one container; an ICO file holds exactly one.
Any PDF bigger than 256 pixels a side. That is not a setting, it is the ICO file structure: width and height each live in a single byte. The engine scales larger sources down, and only down, so a phone photo becomes a 256 pixel icon.
The PDF text layer. A PDF usually carries selectable text; this path treats the page as a picture and paints it, so what comes out is an image of the page rather than its words.
If a model is going to read the ICO
An ICO file is pixels, and pixels are expensive for a model to read. Handed this ICO, an assistant spends its budget looking at the picture rather than reading it, and it will paraphrase what it thinks it saw. If your PDF is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
PDF to ICO, measured rather than promised
Running PDF to ICO against the real engine with a real file, ImageMagick wrote 161,630 bytes of ICO in 0.1 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 ICO 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 ICO 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 ICO 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 ICO, and what they measured
Among the published routes into ICO, PDF is the second largest output of the 10 measured. The witness files differ, so this ranks the probe run and not your document.
JPG to ICO: 99,422 bytes in under a tenth of a second.
BMP to ICO: 99,422 bytes in under a tenth of a second.
GIF to ICO: 202,814 bytes in under a tenth of a second.
HEIC to ICO: 99,422 bytes in under a tenth of a second.
HEIF to ICO: 99,422 bytes in under a tenth of a second.
What we will not pretend about PDF to ICO
A PDF file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A PDF to ICO run that passes 30 seconds is killed, and the ImageMagick process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
No text is read out of the PDF anywhere on this path. Converting a picture to ICO never turns it into words, whatever the picture happens to contain.
ImageMagick was not saturated at sixteen simultaneous conversions: it held 47.5 a second while latency moved from 0.22 s to 0.34 s, and the service memory did not move off 51 MB. That is the measured headroom behind PDF to ICO, not a marketing number.
One PDF file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
PDF to ICO: what people ask
What actually converts my PDF file to ICO?
ImageMagick does it, 6.9.12, on a machine we rent and watch. Not a browser trick and not somebody else service: the PDF file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept afterwards.
How long does PDF to ICO take?
On the file the probe used, ImageMagick took 0.1 seconds and wrote 161,630 bytes of ICO. 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 30 seconds the run is stopped.
What do I lose going from PDF to ICO?
The one to know about first: Scalability. PDF describes shapes, ICO stores a grid of pixels, and the engine rasterises at 150 dots per inch on the way across. That density is a deliberate choice: at the 72 dpi ImageMagick defaults to, an A4 PDF page comes out 595 pixels wide and reads as a failed conversion. Enlarge the ICO afterwards and it blurs, because there is nothing left to redraw from.
Can I use the ICO as a favicon?
Yes, and it is the usual reason for this pair. ICO is the Windows icon container, and the size of the PDF you send decides what you get: anything over 256 pixels a side is scaled down to fit, anything under is left exactly as it is, so a 32 pixel PDF makes a 32 pixel icon. Send a square one if you can.
Is PDF to ICO free?
There is a free allowance every month, and one PDF to ICO 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 ICO.