SUMMARY
Okular highlights are the only PDF reader highlights not Gmail compatible (Okular highlights are not visible in Gmail but all other products do not have this problem).

Because all other PDF Reader/Editors highlights do not have this problem,
Google does NOT consider this a Gmail problem since it only exists with Okular highlights.

STEPS TO REPRODUCE
1. Highlight any PDF using Okular.
2. Email Okular highlighted pdf via Gmail.
3. 

OBSERVED RESULT
No Okular highlights are visible.

EXPECTED RESULT
Okular highlights should be visible (like they are with any other PDF reader or Editor) but they are not. The problem happens 100% of the time so users are unable to use Okular since highlights are not visible in Gmail.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
Currently using multiple Linux Mint 20 Cinnamon systems but the problem has existed with all known previous versions.

Thank you for your support.