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Turn on Import Options when placing the PDF, there you can set the background to Transparent or not.
we did that allready, it dosent works...
Then: if (1) your imported PNG file have a transparent background, but (2) the PDF you created from them does not, it got lost in the PDF conversion.
This is the same PNG image, placed into an InDesign and exported as PDF. Left: compatible with Acrobat 4 (PDF 1.3), which does not support "live" transparency. Right: compatible with Acrobat 5 (PDF 1.4), which does.
Even though the image background is transparent in the InDesign document – the blue and orange rectangles appear 'below' and 'on top' of the image in both versions –, the transparent background itself cannot be exported as an older PDF version.
There is nothing you can fix in these files; you need to recreate them, and make sure to save them with a setting that does allow transparency.
What kind of PDF is it? If it is PDF/X1-a or PDF/X-3 or any printed PDF or any postscript related PDF and you have used transparency or drop shadow you will get a white background.
Use only PDF/X-4 if you place PDFs in InDesign.