PDF Chapter Ordering

So to give an example I have download CH 1-20 of Switch Girl from MangaReader and had it download to a single file. I also had it output to a PDF as well. However, the PDF is ordered incorrectly. The order being CH 1,10,11,12...19,2,20,21

Like that. Not sure I'd you are aware or maybe it is just that source. However I have to say you have made an amazing software and I really appreciate it.

Thanks!