How to Prevent Cropped Linked PDFs from Revealing Hidden Information After Export in InDesign

I have used indesign for many years and never had this issue. I link pdf drawings / scans of images into my indesign document and crop to the information I want. Sometimes the information cropped out could be sensitive information we do not want to share.

Exporting the document appeared fine on my machine but when sent to an external party it appeared with the pdf images uncropped and showing additional information all over the pages. 

This only happened with the pdf links. 

Are there any settings that could have made this happen or what can I do to ensure it does not happen? As it appeared correct on my machine I had no idea that after export to pdf it retained the cropped information and just want to ensure I create flattened pages from now on.

Thanks!

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