Final answer:
When exporting an Illustrator file to PDF, transparent objects can be flattened depending on the PDF standard used or the unsupported effects in the file. The objects may need to be flattened for compatibility reasons or to maintain their appearance. Different export options or manual flattening can be used to control the outcome.
Step-by-step explanation:
When an Illustrator file is exported to PDF, transparent objects can be flattened due to certain conditions. One possible cause is using a PDF/X-1a or PDF/X-3 standard when exporting, as these standards require flattening of transparency. Additionally, if the file contains blending modes, overprinting, or transparency effects that are not supported by the PDF format, the objects may be flattened to maintain compatibility.
For example, if an Illustrator file has a transparent object with a blending mode that is not supported in the PDF format, such as 'Overlay,' the object will be flattened during export to maintain its appearance.
To prevent unwanted flattening, you can try exporting the file using a different PDF standard that does not require flattening of transparency, such as PDF/X-4. Alternatively, you can rasterize the transparent objects before exporting to PDF or flatten them manually within the Illustrator file to control the appearance of the flattened objects.