The Cover and Text PDF files must be the same size as the book trim size being ordered. If PDF book files are supplied as 8.5 × 11, but you want a 5.5 × 8.5 book trim size, then the files will have to be reduced, which reduces the fonts and all the elements in the PDF files. Reducing the file size is not recommended, because it will make the type small and hard to read plus large margins.