Could probably be down to the PDF reader they are using. 3rd Party PDF readers are not great - like foxit reader, chrome reader, mac preview, and others.
Only Adobe Acrobat or Adobe Reader can show every PDF according to its standard.
You could create a layer on the top in an INDD which cover everything, write on it that the people have to use Acrobat or Reader and make this layer invisible in InDesign before exporting it to PDF.
When someone is using a browser which does not support the standard he will see this layer.