When it was first introduced by Adobe, transparency was (and to some degree, still is) a bane to the prepress production community. Transparency gives creatives the ability to incorporate elements into their designs that previously were not possible. However, if transparency is not used judiciously in a document, problems can arise when the job is printed on press.
One of the most common situations associated with transparency is when it is used in conjunction with fonts—depending on how they were used in the design, the fonts could be converted to outlines. The result of fonts being converted to outlines is that the text becomes more bold, or “fatter” (see image below). While you may not be able to see this easily on an inkjet proof or PDF file, you will most certainly see this when the job is output and printed on a digital or affset press. The solution to this problem is fairly simple: To keep fonts from converting to outlines when text is used with transparency, make sure that the text is on the top layer of all objects in a transparency stack.
One of the best diagnostic tools for checking to see if fonts will be converted to outlines in Adobe InDesign and Illustrator is the Flattener Preview palette (Window>Output>Flattener Preview and Window>Flattener Preview, respectively). By selecting the various options under the Highlight pull-down menu, you can preview which objects are transparent on a page, which objects will be affected by transparency, and which fonts (if any) are converted to outlines.