The image sent with a subscription in Tableau 8 Server is a great first start, but alas the image is always of poor resolution when the dashboard is set to Automatic. It would be much better to be able to specify an image size (x by y) and an option for PDF (and its size- perhaps rather than x by y but 'letter, legal, A4', etc) at the time when you create the subscription. By specifying the size then the image can become usable and by allowing PDF an attachment option can be provided as well.
--> Community Manager update [Ciara Brennan]
Specifically related to PDF subscriptions, please try out the 2019.3 beta version
Coming Soon: New features in Tableau PDF attachment to subscriptions ( Feature 110653 )