The character sheets are in landscape format, while the book is portrait. This is obvious, but more obvious and inconvenient is when reading on tablet or even screen with fit-width option is turned on. The different page sizes in the file cause the whole book to not spread between screen edges and on tablet devices such as iPad this results in super annoying experience when scrolling - the page is moved not only vertically but also horizontally if you are not precise.
In general, I remove/rotate the problematic pages with Adobe Acrobat, but the pdf is locked. From design perspective rotating the sheets will look not so great, removing them - not good idea at all, redesigning in portrait - I have no idea if you have such version.