How Preflighter works

What happens to your PDF between upload and preview, which software does the work, and where the limits are.

From upload to preview

Upload. The PDF goes over HTTPS to my own server. It is not sent to any cloud rendering service. How long it stays there is on the privacy page.
Colour profile. If the PDF has an output intent (PDF/X), the embedded ICC profile is used. If only the name of the printing condition is given, it's matched to a profile from the official ICC profile registry. Without an output intent the preview uses FOGRA39 (coated paper) and says so.
Separations. The renderer draws each page straight into ink channels: cyan, magenta, yellow, black and up to 16 spot colours, each as its own plate. Overprint is applied the way a press would: an overprinting object doesn't knock out the ink beneath it.
Overprint comparison. For the “with / without overprint” view the server renders the page twice, with overprint simulation on and off, so you can see exactly what overprint changes.
On your screen. The browser combines the plates you have switched on into the on-screen colour using a lookup table built from the ICC profile. Total area coverage (TAC) and the densitometer readings are calculated from the same plates, so all views show the same ink.
Checklist. Bleed, trim size, image resolution, fonts, PDF/X and output intent are read from the PDF structure. Checks that depend on rendering are collected page by page. Until every page has been seen, a check says “not verified” instead of showing a green tick.

The core is open source. Nothing is a black box you'd have to take on trust.

Poppler	PDF rendering and separations. GPL, runs only on the server and is not distributed. Built with up to 16 spot colour channels and with security fixes applied.
Little CMS (lcms2)	ICC colour management. MIT licence.
ICC profiles	From the official ICC registry at color.org, or embedded in your PDF.
Python, FastAPI, Pillow	The server around the renderer. Open-source licences.

It's a soft proof, not a contract proof. What you see depends on your screen. For colour-critical jobs, ask your printer for a certified proof.
It shows, it doesn't fix. Preflighter doesn't change your PDF. Converting colours, adding bleed or fixing fonts has to happen in the program you exported from.
Up to 16 spot colours per page. If a page has more, the tool tells you which ones it couldn't show.
Transparency is shown as rendered, not as a specific RIP would flatten it. Files with transparency get a note about that.
Resolution adapts to page size. A billboard opens quickly, but at 100 % zoom its preview is coarser than the original.