One of the most common complaints of designers or print service providers when previewing and printing transparency from InDesign is that a transparency effect like a drop shadow doesn't display or print correctly. Instead, a white box appears behind the transparency effect.
When you probe into how the transparency effect was created, there is usually one common element: The designer is applying the transparency effect so it interacts with spot colors. The illustration below shows text with a drop shadow placed over a blue frame. The background frame above is colored as a Pantone color; the one below was colored CMYK.
Whenever transparency is used in InDesign, it must be flattened for printing. Printers (and the PostScript language used for printing) don't understand transparency. So if you were to take the example shown above and export it to PDF using one of the two PDF/X PDF presets, Acrobat 4 compatibility is automatically selected, and the PDF is flattened.
Many print service providers prefer receiving files with transparency preflattened, and that's what these presets do.
If you then open the PDF in Acrobat and Reader, this is what you will initially see?what I call the "white box effect."
So what are you going to do? One option is to not use spot colors when there are transparency interactions. We can see that that worked above. But you can use spot colors if you turn on overprinting. In Acrobat 6 or 7 Professional, choose Advanced > Overprint Preview. In Adobe Reader 7, you can also turn on overprinting in Page Display Preferences. Then you'll see the transparency effect the way that was intended (see below):
This means that in order to print properly, overprinting must be turned on. This is the only way that the transparency flattener in InDesign (and the other Adobe Creative Suite applications) can properly render transparency mixed with spot colors. If you're printing a proof on a printer which doesn't understand overprinting, you can turn on the Simulate Overprint option. In InDesign, this is found on the Output panel when you select one of the composite printing options (below):
If you're sending your file to a print service provider, be sure to tell them to turn on overprinting on their RIP when printing your job. Many service providers by default have this option turned off, but to print with spot colors and transparency, it must be turned on. This is the only way you'll eliminate the white boxes!