In our online PubHTML5, the page limit of PubHTML5 is no more than 500 pages while the size of which is no more than 150Mb. In addition, except the Free User, others have no limit on the page of flip book.
However, on our Pubhtml5 desktop client, there are no limit to the page and size presently. Thus, as a recommendation, it's better for you to convert large documents on our PubHTML5 desktop client.