When the first page of a file is larger than the following pages, and the View options "Continuous" and "Facing Pages" are both checked, there is an excessively large gap between all pages after the first set. The gap also exists when ""Page Breaks" has been unchecked. Whether "Book Mode" is checked or not also makes no difference. (In particular, this is a problem with files from the popular humanities database JSTOR, because the first page of these files, which contains the citation information, is almost always larger than the pages that follow.)
Enclosed is a screen-capture demonstrating the effect.
Screen-capture showing gap and options
This is not a bug, the width of the colume is (apparently) set by the widest page, no matter which one that is.
Apart from that, display of the PDF is PDFKit's responsibility.