>> "preflight" in Acrobat Reader,
In Reader? Are you sure?
>> "overprint on"? All fonts? - How do I solve this?
If you're really don't want to have black text to be overprinted than you're need to apply some non-default black color to this text and change attributes. InDesign do overprint for swatch Black 100% always.
https://helpx.adobe.com/indesign/user-guide.html/indesign/using/overprinting.ug.html
Quote:
To knock out black objects in InDesign, you must prevent the black swatch from overprinting. Unlike most color swatches, which knock out by default, the black swatch overprints by default, including all black strokes, fills, and text characters. The 100% process black appears as [Black] in the Swatches panel. Knock out black objects by either deselecting the overprint default in Preferences or by duplicating the default black swatch and applying the duplicated swatch to color objects that knock out. If you disable the overprint setting in the Preferences dialog box, all instances of Black knock out (remove underlying inks).
Note:
It can be cheaper and easier to have the print shop overprint process black on the press.
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Choose Edit > Preferences >Appearance Of Black (Windows) or InDesign > Preferences >Appearance Of Black (Mac OS).
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Select or deselect Overprint [Black] Swatch at 100%.
Quote about attributes:
You can overprint strokes or fills of any selected paths using the Attributes panel. An overprinted stroke or fill doesn’t need to be trapped, because overprinting covers any potential gaps between adjacent colors. You can also overprint a stroke to simulate a trap (by overprinting a color you’ve manually calculated as the proper combination of two adjacent colors).
Note:
Overprint [Black] Swatch at 100% does not affect tints of [Black], unnamed black colors, or objects that appear black because of their transparency settings or styles. It affects only objects or text colored with the [Black] swatch.
Remember, never say you can't do something in InDesign, it's always just a question of finding the right workaround to get the job done. © David Blatner