/01 What a PDF preflight check does
Preflight is the check a print shop runs on your file before it goes anywhere near a press. A PDF preflight tool looks for the problems that make a print job fail: wrong page sizes, missing bleed, unembedded fonts, low-resolution images, and colours that will not survive the trip from screen to paper.
The point of checking early is simple. A problem caught before sending takes a minute to fix. The same problem caught after printing takes a reprint, and after a press run it takes money that is already spent. This tool runs the full check inside your browser, so your document never gets uploaded to anyone's server.
Drop your PDF in, and it reads the file page by page and reports what it finds. Issues are sorted by how serious they are, so you know what must be fixed and what is merely worth knowing.
/02 The checks that catch real problems
The geometry checks look at the boxes inside the PDF that define the page. The MediaBox is the full page area, the TrimBox is the finished size after cutting, and the BleedBox is the extra area that gets printed and then trimmed away. If the bleed is missing or too small, anything meant to reach the edge of the page risks a thin white sliver on one side after trimming, because no cutting is ever perfectly exact. Most printers want at least 3mm of bleed.
The font checks make sure every font in the file carries its own letter data. A font that is not embedded gets swapped for whatever the printer's system has, and your carefully chosen layout reflows into something else. The image checks look at how many pixels each picture really has once spread across its final printed size, because an image below about 300dpi looks soft and blurry on paper even when it looked fine on screen.
The colour checks flag images left in RGB when the job is heading for a CMYK press. The conversion happens either way, but an unmanaged conversion shifts vivid colours, and greens and oranges suffer most.
/03 Errors, warnings, and what to do about them
Not every finding means disaster. An error means the job will likely fail or print wrong, and you should fix it before sending. Missing bleed on a page with edge content is an error. A font with no embedded data is an error. A mixed page size in a document that should be uniform is an error.
A warning means the job will probably print, but with a known risk worth a second look. Images at 250dpi often survive. A dark photo at low resolution will not. Colours slightly outside the printable range may come back duller than expected. Warnings are where judgement comes in.
Read the report, fix the errors, and use the warnings to decide what you can live with. The report is organised page by page, so fixing things is a matter of working down the list rather than hunting through the file by hand.
/04 Why it pays to check before you send
Every print shop runs a preflight of its own. When their check finds problems, the job pauses, and you get an email asking for a new file. That is the good outcome. The bad outcome is a job that passes a quick glance, goes to press, and comes back wrong, because nobody caught the missing bleed until the trimmed edges showed white lines.
Running the same check yourself first means you send a file that passes. It also means you understand your own document better, because the report tells you things most people never look at: which fonts are actually in the file, how big each image really is, and whether all the pages share one size.
And because this tool never uploads your document, you can preflight files that contain things you would rather not hand over: contracts, client work, anything under embargo. The check happens on your machine, the file stays on your machine, and the report is yours alone.
/05 What makes a PDF print ready
A print ready PDF is one a printer can run without asking you to fix anything. Most print shops look for the same things: the correct final page size, 3mm of bleed on any artwork that runs to the edge, every font embedded, images at around 300 pixels per inch at their printed size, and colours set up for the print process rather than only for screens.
Missing any of these costs time. The printer may send the file back, charge for correcting it, or print it as supplied, which is how you end up with white slivers at the edges, fonts replaced by substitutes, or blurry photos. Preflighting catches these problems at your desk, where fixing them takes minutes.
Drop in your PDF and this preflight check reports page sizes, bleed, colour spaces, fonts and image resolution, flagging errors and warnings so you know what to correct before the file goes anywhere.