It's going to be much slower with a file that size, but it should still work. I just tested a 50 MB PDF with 400 pages and was able to insert another PDF and make changes to the order. If your file is read-only or has some other property that is causing issues with MobileSheets writing to the file, then that's going to be a problem when you insert another PDF into it. If you just change the page order and click "Save", does that work?
Mike
Mike