For portability it is probably better to send PDF.
The OpenDocument format may contain a “Thumbnails” folder but relatorio does not generate any and even remove them. So I guess it is the mail client that generate it wrongly.
But I can see that OpenDocument has a setting name SaveThumbnail which could be set to true. I do not know the implication of such settings if there are no Thumbnail in the file (I hope it is just ignored).