Sometimes you have multiple PDF files and don't know how to print them together in a single bound book. In such case you can simply compress multiple files in to zip or rar format file and use that for placing order.
These files when uploaded then total number of pages are calculated by considering pages in each file. All files inside zip or rar files are printed in the order when sorted by name. If sort by name is not the giving a desired order then rename files and put numbers in front of it and it will printed in numbered order.
If there are many small files available in your multiple folders then you can simply compress root folder and upload that compressed files to be printed on single merged book. Just remember to include only PDF files in your archival either zip or rar format and enjoy merge PDF printing.