Re: “There is no easy way.”
If you wish to sell a pdf then open a free ECWID account, enter your paypal info and upload your pdf. In ECWID click “generate code” and paste that into an HTML stack. Publish. Done.
Within the ECWID control panel adding tax settings for digital or physical goods etc., etc., are very straightforward.
I sell downloadable wall art and shippable gifts without any of the stacks you mentioned, only the default stacks that come with Stacks, nor do I have any ECWID related stacks. I have an ECWID business account because I have more than 100 products.
Hope that helps.
EDIT: I have a free download on that Wall Art page if you wish to test the cart from customers perspective.