Your duplex scanner did fronts first, backs second, but somehow the backs are in reverse order. Now you have a 60-page PDF with fronts 1-30 followed by backs 30-1. Reading it requires a calculator.
Fixing the order is exactly what Flint is for.
Diagnose the pattern
Open reorder PDF and look at the thumbnail grid. Identify the pattern: maybe fronts are 1-30 (correct order) and backs are 31-60 (but representing pages 30-1). Knowing the pattern lets you fix it in batches rather than dragging individual pages.
Two common fixes
Pattern A: alternating fronts and backs got separated. Solution: split into odd and even files, interleave back with merge. Pattern B: backs are in reverse. Solution: in reorder PDF, select pages 31-60, right-click 'reverse selection', then interleave them with 1-30 by dragging. Pattern C: random order. Solution: drag pages individually using thumbnails as your guide.
Interleave with drag-and-drop
Once backs are in the right relative order, you need to interleave them with fronts. Drag back-page-1 between front-page-1 and front-page-2. Drag back-page-2 between front-page-2 and front-page-3. Tedious for many pages — easier to split, then merge in alternating order. For under 20 pages, drag is fine.
Prevent next time
Configure your scanner to do duplex correctly: 'simplex duplex' or 'true duplex' depending on hardware. Most modern scanners can interleave automatically when set up properly. Manual reordering once is OK; doing it every time is a scanner-config issue.
FAQ
Can Flint detect and auto-fix duplex flip?
Not automatically. You select the affected pages and reverse them manually.
What if some pages duplicated?
Delete duplicates before reordering. Easier to fix the cleaner set.
How do I reverse a range?
Select the range, right-click, 'reverse selection'. The pages flip order in place.
Will the original file be modified?
No. Flint outputs a new file. Original on disk is untouched.
Diagnose, batch-fix, save. Reorder scanner output.