Does this sound farmiliar? Your scanner doesn't support duplex scanning, hence you have to tediously rearange the order of the scanned PDF manually after each scan.
At first you tried manually drag and drop in Preview to reorder the pages, and then you gave up after 10 pages. It just feel impossible to manually rearrange dozens of pages in Preview.
You googled a bit and found some suggestion to use a PDF tool shareware that cost quite some amount of money, it certainly doesn't feel worth to purchase a full suite of PDF tool just to reverse page order! Hah, Don't even get us started on the pricing of Adobe Acrobat.
Like you, I got fed up, and thus decided to make this app in a whim.
I hope this app is useful for you for handling scanned PDF!