Dupleasure (from duplex and pleasure) reorders the pages of a PDF file to recreate a duplex-style scanned PDF. It interleaves the first half of the pages with the second half of the pages, simulating the front and back sides of a physical document.
For example, you feed a physical document of three sheets into a scanner: First you'll have it scan the three front pages, then you'll turn over the stack of paper and have it scan the three back pages of the document. The PDF you'll get will contain the physical pages in the following order: 1, 3, 5, 2, 4, 6. Dupleasure will take this PDF and put the pages in the order you want to read them: 1, 2, 3, 4, 5, 6.