If you have created a product on Gumroad and want your customers to be able to take full advantage of the Gumroad Library app (for iOS and Android!), it is important to be aware of the types of files that can be accessed in the Gumroad Library app.
For your convenience, we've assembled those filetypes here:
Optimizing a PDF for Mobile:
Set the page size to 6x9" and using a 12pt font for the body. The defaults tend to make PDFs hard to read on mobile because they are generally optimized for print.