[Cooper G]

Hi! Cooper from the Ecwid team here.

Yes, this is a known issue: different email client software process the HTML messages in their own ways. In case of the Thunderbird, looks like this is caused by the option "Display Attachments Inline". See it on the screenshot: http://take.ms/7Xfwg
So simply disable that option.

As for the invoices in PDF, we have it in the feature request list, but I can't give you any ETA for its implementation. If you want, I can add your email to the feature request list so that we let you know when this feature is ready.