Structured ebook content becomes a sequence of pages
FB2 represents book text and metadata in a structured form, while PDF fixes the composition to page dimensions. This makes page references and print layouts possible, but removes much of the natural reflow readers expect from an ebook.
Check the title, author, language, and chapter divisions. A structured source can contain annotations or notes that should remain associated with the right passage; verify their placement rather than assuming the page renderer will arrange every element as intended.