When uploading or saving files always make sure that the number of digits in the file has the same amount as the highest page, so if it's 20 pages all pages should be 2 digits long; 01,02, etc. If it's 150 pages all pages should be 3 digits long 001,002.
This prevents pages from being out of order because 11 comes before 2 but not before 02 when it comes to file names.