HI Uwe,
This is the link to one of our PDF, that Shows blue Background, opened in Firefox.
https://www.recom-power.com/pdf/Innoline/R-78Cxx-1.0.pdf
i hope you can find a solution, which showes the datasheets correct. Thank you very much.
Regards, Magdalena
Hi Magdalena,
I think the culprit is the logo on page one saying "3 Year Warranty".
The gradient of the logo expanded to the whole page when seen with e.g. Firefox Reader.
You can work around that issue if you export to PDF Version 1.3 that is flattening transparency.
If you want to be on the save side also convert all colors to sRGB. Set your Transparency Blend Space to RGB.
I placed your PDF in a new InDesign document. After export I opened it in Firefox browser and had no problem with page one:

Here the one from your website:

How I did it. From my German inDesign CC 2018.1 on Windows 10.
Compatibility set to Acrobat 4 (PDF 1.3)
In the second tab not showing I turned off image compression or downsampling at all.

Convert to Target Profile, Target is sRGB and Simulate Overprint clicked:

Transparency Reduction set to "High Quality". You could do your own set if you like.

Regards,
Uwe