A digital sourcebook is a completely digital version of one of the published books, such as the Player's Handbook or an adventure like Curse of Strahd. While it is not a PDF, the book is re-created in digital format on D&D Beyond in the Sources section of D&D Beyond, and contains hyperlinks for ease of use.
If you have not purchased the digital book on the Marketplace, you will only be able to see the table of contents to get an idea of what is in the book.
If you do purchase the digital book on the Marketplace, you'll be able to read the book in full, as well as unlock all of that book's content for use throughout the toolset where applicable, including the Character Builder, Encounter Builder, and the Maps 2D VTT.