When uploading a document, one or more preview pages are automatically selected, based on the size of the document, and are displayed to potential customers. The amount of selected pages can be changed. Go to uploads → click on the three dots next to your document → edit.
When deciding how many pages you want to show to your potential customers, it is important to remember that the customer bases their decision to buy on the title, available information such as description and what the document looks like on the previewed pages. So give it a good thought!
Look at the support page : What do I show on my preview pages? For more information about this topic.