What the problem actually is
A transparency effect (opacity, soft shadow, blend mode) exists as a "live" instruction inside the design file. When the file is exported or processed by the print system, that transparency has to be interpreted and turned into something the press can actually reproduce — and it's at this step that differences from what's shown on screen can appear.
Native transparency and flattened transparency
Native transparency stays "live" inside the PDF (it requires a sufficiently recent PDF version and a compatible print engine). Flattened transparency is instead converted, already at export time, into equivalent opaque shapes — necessary for older workflows or RIPs that don't support native transparency.
Why it depends on the PDF format and the RIP
There's no transparency setting that's always safe: it depends on the required PDF standard (the various PDF/X versions, for example, handle transparency differently) and on how the printer's specific RIP interprets it. The same setting that works perfectly with one supplier can produce different results with another — it always needs to be confirmed, never assumed.
How to recognize flattening errors
Flattening can split overlapping objects into many smaller fragments, sometimes visible as thin lines or seams along the edges on certain print engines. It can also alter the perceived color in areas where transparent objects overlap, and it's especially delicate when Pantone spot colors interact with transparent elements.
A practical check
In Adobe Acrobat Pro, the Print Production tools include a transparency flattening preview, useful for spotting unwanted splits or color shifts in advance. Document Properties also shows the file's PDF compatibility version, a detail worth confirming with the printer before sending.
Common mistakes to avoid
Exporting the PDF with the software's default transparency settings, without checking what the specific printer requires. Using soft shadows or light effects over Pantone spot colors without testing the result. Assuming the on-screen preview exactly matches what flattening will produce in print.