Great question! It's important to note that some blank pages are completely normal when formatting a book according to the standard publishing conventions, and some pages will always have a blank page to adhere to these standards:
The Copyright page will always begin on the left.
The Table of Contents will always begin on the right.
The Title Page will always begin on the right.
Chapter 1 (or the first page in the body section) will always begin on the right.
Beyond this, there are two possible reasons your PDF may export with blank pages:
Your book is set to start new chapters on a specific side of the page, or
There are invisible trailing spaces at the end of a chapter, creating a blank page
If your book has completely blank pages showing up before a new chapter or chapters in your book, these have been added to ensure that the following page or chapter begins on the right or left side of the spread. To adjust this:
Click the three dots beside the title Body in the left navigation panel
Choose Begin On > Left side (to reset the programming)
Click Begin On again, and this time choose > Either side to get rid of all blank pages. Keep in mind that these options will not affect the pages mentioned above.
Hope this helps!