How do I flatten overprint?

When overprint is not desirable in a workflow or when output devices don't support it, how can it be avoided? Overprint flattening is a prime way to deal with that issue (even if as a solution it is not always desirable, see "overprint flattening" for more background)

Rendering

While not overprint flattening per sé, having an engine that understands overprint create an image of each page of a PDF document is in fact a way to get a similar effect. You end up with images which will print on any device, regardless of their support for overprint. Possible tools could be something like Adobe Acrobat that allows you to save images, or a tool such as callas pdfToolbox.

The "Simulate overprinting" option effectively makes sure that pdfToolbox will take overprinting objects into account as it is rendering your PDF document into images.

Overprint flattening

Adobe PDF engine embedded in Adobe Acrobat and a number of other tools such as callas pdfToolbox, supports transparency flattening and, as a part of that, overprint flattening. In Adobe Acrobat, amongst other techniques, you can do a "Save as" and select "Optimize PDF". This brings up a dialog window with transparency flattening options:

Notice the "Preserve Overprint" setting at the bottom of the transparency flattener options. Remark that this is a negative setting; you will have to set "Preserve Overprint" to "False" in order to remove overprint from your optimized PDF file.

In callas pdfToolbox the same operation can easily be automated because it is available as a fixup and can be included in preflight profiles. You can download the following preflight profile that contains an example overprint flattening fixup you can try: