I had a detailed discussion on the Arch Linux bug reporting site about this issue.
One of the packagers and myself did some pdf font export testing.
He did it, I replicated his results.
It seems the issue with text transparencey in epub to pdf exports may be related to font setting not being specific in the pdf export settings.
The Arch packager suggested this is an upstream issue in the original Calibre package which should be looked into.
While I am now using a Calibre install, direct from the Calibre web site, which does not seem to contain this issue, it might still be worth looking into this.
The discussion on the Arch bug reporting web is shown here
https://gitlab.archlinux.org/archlin...bre/-/issues/4
Calibre is a great package, thank you for it.