You can choose to generate a preview of only a subset of your book, if you are in Dropbox, GitHub, or Bitbucket writing mode.
This only produces a PDF, because the whole point is to generate the book faster than usual.
To do this, create a file called
Subset.txt in your book's
Then, go to the preview page for your book in Leanpub, and select "SUBSET PREVIEW".
Then, click the blue "Create Preview" button.