If you don’t have access:

We show the first 2 pages for every document, unless the document’s size is actually less than 2 pages, in which case we don’t show a Look Inside. (e.g. reference work entries).

If you do have access:

We show the whole document in the look inside. This counts as a download if the user browses to page 3 or beyond.

We have done the rasterization to make images out of every page of the PDF document for almost all the documents, but for very new ones you may find that this process has not been completed yet, in which case you will get the message when you open the Look Inside “Free Preview not available” (in English and in German).