You can choose to generate a preview of only a subset of your book, if you are in the Dropbox or the GitHub writing mode.

This is a special feature that is only used when an author wants to preview a single chapter.

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 manuscript folder.

Then, you type the filename of the file you want to preview in the Subset.txt file, just like you type filenames for the full book into the Book.txt file.

Then, go to the preview page for your book in Leanpub, and select "SUBSET PREVIEW".

Then, click the blue "Create Preview" button.