Creating a single pdf document from multiple scans from a two-sided print doc. What is the easiest, quickest way to rearrange the pages into their proper order?

I am currently attempting to create a single pdf file from several scans of groups of pages from a document printed on both sides of the page. The manual is approx. 80 pages and I did not want to scan each page independently. Is there a way to move the pages around once they are all combined into a single pdf? All the odd pages are in order. However, needed two other scans for even pages abd they are at the end after the odd of pages.