More and more print shops do a “digital proof” instead of a real physical proof copy like you’d get in the olden times.
In other words, you send them a pdf of the thing you want printed, and they send you the pdf back.
What is this ritual intended to accomplish?
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The pre-press processing of the PDF format is now so automatic and consistent that what you seen on the screen is very close to what gets printed, especially insofar as what most people care about – names, dates, text, arrangement, basic color correctness etc.
The things that used to cause issues in actually printing, like hairlines, overprinting, ink saturation are either flagged or managed automatically. Color can still be a bit off due to the differences in medium – screen vs paper, but close enough for general audiences with differences predictable by professionals. Professionals still ask for hard copy proofs on occasion, but usually only digital print proofs for offset – which won’t include special inks or varnishes, so they have to be predicted or imagined anyway.
Edit: one thing you might be wondering is if you send a pdf in the first place, do they send you back the same pdf? They do not – they check and remake the pdf for compliance to their printing machines.
Your submitted PDF might include a coloured area with a semi-transparent layer over the top. Of course the actual printing device isn’t going to do semi-transparent overprinting; it just prints a single image. This means that software somewhere needs to convert multiple layers into a single layer, modifying colours as necessary to (hopefully) achieve the intended effect. This is known as “flattening” a PDF document and doing it correctly, in the face of the various colour spaces used for documents and printers, is not simple.
The digital proof PDF you receive will have been flattened and probably processed in other ways. A CMYK printer has four ink colours so you might think it can print 400% coverage, but often they’re limited to 240%; your PDF might need to be modified to suit. You should check carefully to ensure that it looks the way you want.
You can simulate what will come out of a specific printer. Not perfectly, but well enough to at least catch moderate to large issues that the customer might want to have adjusted before printing. The PDF you receive is the result of that “simulation” – it’s not just the PDF you delivered (at least it shouldn’t be).
E.g. one big factor is color spaces and converting between them. A printer has a certain range of colors it can produce. If you used colors outside that range in your document, the result will look different from what you intended. Also, the colors in your document were encoded in a certain color space, and this will need to be converted to the color space used by the printer, and this conversion can change the exact appearance of certain colors (especially if the conversion isn’t done correctly for whatever reason, e.g. because your document didn’t include a specification of the color profile you used). Still, the digital proof will only ever be an approximation of the resulting colors, especially if you’re not viewing the document on a color-calibrated screen.
For higher-stakes printing, a physical proof print is still the gold standard.
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