Janet,
This is primarily a "MODx Question", but also part FoxyCart ...so you’re in the right place!
FoxyCart indeed supports "Downloadables" (eBooks, etc), as well as the shippable products you mentioned, or even subscription based products (recurring payment), etc. The integration you are looking for occurs within MODx framework (Product Pages) but has ties to the FoxyCart system. MODx would be responsible for displaying the correct "link" passing to FoxyCart a new, downloadable cart item prior to Checkout. PayPal is simply your payment gateway integrated via FoxyCart.
Downloadable
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Downloadable products are just like regular products as far as how they’re added, but there is a very important difference in that downloadable products must first be uploaded to your store [FoxyCart]. When you upload your product, you’ll assign a code and a category to it. You’ll use the code and the category attributes, rather than the name and price.
I am very familiar with both MODx and FoxyCart, as they are my preferred solution for all of my e-Commerce projects!
I have not personally worked with Downloadables yet, however the key difference appears to be the way integration must also occur on the FoxyCart side before-hand. Depending on how your developer set you up, you might be able to access your "Downloadable" within your FoxyCart admin section. However, it sounds like this step may not have occurred yet....If that is the case, you would likely need to work on setting up the Downloadable product, mapping it to the correct eBook file (PDF), and updating the Product page/website to reference this new "Downloadable" product code/category opposed to the standard name/price. Of course, I don’t literally mean YOU, unless you’d like to learn the MODx API or FoxyCart system