Problems inserting transparent PDF page into the PP presentation

I have just stated that if I insert a (partially) transparent PDF page in a PP presentation, the Presi secure and re-open, the image used (PDF page) now in the background is no longer white but black.
I can not prevent this by assigning a background color to the image. I assume that the PowerPoint converts the transparent image while backing up, filling up all transparent elements with black. Can this be? Is there a trick to prevent this conversion? Or am I forced to work with JPEGs / PNGs?


Please help....


I didn't find the right solution from the Internet.
References
https://answers.microsoft.com/en-us/msof...74-19b0d3eff448
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