Luis,
I was a little too optimistic when I wrote that your file gave no problem. When you open it in preview and select export as PDF, then you should compare the color of the original PNG file and that of the PDF and observe that the blue color in the PDF is mixed with grey. If you would increase the transparency of your blue box in the PNG much more, you will observe that the color in the exported PDF will become more grey. Inseting your PNG in a Powerpoint or Keynote presentation and converting to PDF gives the same problem. So back to square one !
The only workaround I found with Powerpoint slides containing PNG images with transparency who give grey color when converted to PDF, is first to export that whole slide as PNG (it then includes the background of the slide) and then insert that PNG in the problem slide before converting to PDF.
Hence the problem is only with PNG images with transparency features on a transparent background, so that solves it.
Your suggestion of opening the PNG in Preview and saving it again as PNG only works if you uncheck te Alfa checkbox, but then you lose the transparent background, and that is not a solution when you want to use that image in a document.
Rogier.
