when I open .docs documents directly from Thunderbird emails i get the error message from my WPS Offiice Writer:

/tmp/mozilla_mx0/<doc name>.docxInvalid file.

The same behavior ocurrs when I use Libre Office. When I save the doc on the desktop then I can normally open it.
PDF docs open nromaly with Ocular.

How can fix it?

This is a Thunderbird/WPS issue so there may not be as much experience with this in a KDE forum, but I'd look at Thunderbird's file association settings, perhaps the command string there is wrong.